diff --git a/CMakeUserPresets.json.template b/CMakeUserPresets.json.template
index 3dea9bb..5354427 100644
--- a/CMakeUserPresets.json.template
+++ b/CMakeUserPresets.json.template
@@ -6,7 +6,11 @@
       "name": "win-local",
       "displayName": "Windows • Debug (local — build dir outside OneDrive/spaces)",
       "inherits": "windows-debug",
-      "binaryDir": "C:/Users/@USERNAME@/pdfeng-build/win-local"
+      "binaryDir": "C:/Users/@USERNAME@/pdfeng-build/win-local",
+      "cacheVariables": {
+        "PDFENGINE_WITH_PDFIUM": "ON",
+        "PDFENGINE_WITH_QPDF": "ON"
+      }
     },
     {
       "name": "win-local-pdfium",
diff --git a/Testing/Temporary/CTestCostData.txt b/Testing/Temporary/CTestCostData.txt
new file mode 100644
index 0000000..ed97d53
--- /dev/null
+++ b/Testing/Temporary/CTestCostData.txt
@@ -0,0 +1 @@
+---
diff --git a/background_region.png b/background_region.png
new file mode 100644
index 0000000..57c2879
Binary files /dev/null and b/background_region.png differ
diff --git a/bindings/CMakeLists.txt b/bindings/CMakeLists.txt
index d95d266..6c9ac89 100644
--- a/bindings/CMakeLists.txt
+++ b/bindings/CMakeLists.txt
@@ -6,7 +6,7 @@ find_package(pybind11 CONFIG REQUIRED)
 # Declare the python module target. We name the target pdfengine_py to avoid
 # target collision with the static C++ library pdfengine, but we set the
 # OUTPUT_NAME to pdfengine to produce the correct importable module.
-pybind11_add_module(pdfengine_py python/pdfengine_py.cpp)
+pybind11_add_module(pdfengine_py python/pdfengine_py.cpp $<TARGET_OBJECTS:pdfengine>)
 
 set_target_properties(pdfengine_py PROPERTIES
     OUTPUT_NAME "pdfengine"
diff --git a/debug_preview_region.png b/debug_preview_region.png
new file mode 100644
index 0000000..97b28af
Binary files /dev/null and b/debug_preview_region.png differ
diff --git a/engine/CMakeLists.txt b/engine/CMakeLists.txt
index 512b7de..587962c 100644
--- a/engine/CMakeLists.txt
+++ b/engine/CMakeLists.txt
@@ -14,6 +14,7 @@ add_library(pdfengine OBJECT
     src/parser/content_stream_parser.cpp
     src/parser/decoration_builder.cpp
     src/text/selection.cpp
+    src/text/text_layout_engine.cpp
     src/fonts/face/font_face.cpp
     src/fonts/face/free_type_manager.cpp
     src/fonts/loader/font_resolver.cpp
@@ -32,6 +33,30 @@ add_library(pdfengine OBJECT
     src/fonts/pdf_fonts/encoding/encoding.cpp
     src/fonts/pdf_fonts/encoding/tounicode_parser.cpp
     src/fonts/pdf_fonts/encoding/cjk_collection_db.cpp
+    src/parser/pdfium_loader.cpp
+    src/parser/pdfium_document.cpp
+    src/parser/pdfium_internal.cpp
+    src/parser/pdfium_reflow.cpp
+    src/parser/pdfium_page.cpp
+    src/parser/pdfium_page_model.cpp
+    src/parser/pdfium_fonts.cpp
+    src/parser/pdfium_edit_session.cpp
+    src/parser/pdfium_edit.cpp
+    src/parser/pdfium_edit_replace.cpp
+    src/parser/pdfium_edit_reflow.cpp
+    src/parser/pdfium_edit_annotations.cpp
+    src/parser/pdfium_edit_pages.cpp
+    src/parser/pdfium_edit_images.cpp
+    src/qpdf/qpdf_extractor.cpp
+    src/qpdf/qpdf_font_extractor.cpp
+    src/qpdf/qpdf_writer.cpp
+    src/qpdf/qpdf_resource_resolver.cpp
+    src/core/image_decoder.cpp
+    src/parser/lexer.cpp
+    src/parser/parser.cpp
+    src/parser/content_builder.cpp
+    src/serializer/content_serializer.cpp
+    src/serializer/ast_serializer.cpp
 )
 add_library(pdfengine::pdfengine ALIAS pdfengine)
 set_target_properties(pdfengine PROPERTIES POSITION_INDEPENDENT_CODE ON)
@@ -56,21 +81,6 @@ target_link_libraries(pdfengine
 )
 
 if(PDFENGINE_WITH_PDFIUM)
-    target_sources(pdfengine PRIVATE
-        src/parser/pdfium_loader.cpp
-        src/parser/pdfium_document.cpp
-        src/parser/pdfium_internal.cpp
-        src/parser/pdfium_reflow.cpp
-        src/parser/pdfium_page.cpp
-        src/parser/pdfium_page_model.cpp
-        src/parser/pdfium_fonts.cpp
-        src/parser/pdfium_edit.cpp
-        src/parser/pdfium_edit_replace.cpp
-        src/parser/pdfium_edit_reflow.cpp
-        src/parser/pdfium_edit_annotations.cpp
-        src/parser/pdfium_edit_pages.cpp
-        src/parser/pdfium_edit_images.cpp
-    )
     target_link_libraries(pdfengine PRIVATE pdfium::pdfium)
     target_compile_definitions(pdfengine PUBLIC PDFENGINE_WITH_PDFIUM)
     # PDFium statically bundles its own libjpeg, zlib, etc. which conflicts with vcpkg.
@@ -98,18 +108,6 @@ if(PDFENGINE_WITH_QPDF)
     if(NOT TARGET jpeg)
         add_library(jpeg ALIAS JPEG::JPEG)
     endif()
-    target_sources(pdfengine PRIVATE
-        src/qpdf/qpdf_extractor.cpp
-        src/qpdf/qpdf_font_extractor.cpp
-        src/qpdf/qpdf_writer.cpp
-        src/qpdf/qpdf_resource_resolver.cpp
-        src/core/image_decoder.cpp
-        src/parser/lexer.cpp
-        src/parser/parser.cpp
-        src/parser/content_builder.cpp
-        src/serializer/content_serializer.cpp
-        src/serializer/ast_serializer.cpp
-    )
     target_link_libraries(pdfengine PRIVATE qpdf::libqpdf ZLIB::ZLIB JPEG::JPEG)
     target_compile_definitions(pdfengine PUBLIC PDFENGINE_WITH_QPDF)
 endif()
diff --git a/engine/include/pdfengine/edit_session.hpp b/engine/include/pdfengine/edit_session.hpp
new file mode 100644
index 0000000..c01c2fd
--- /dev/null
+++ b/engine/include/pdfengine/edit_session.hpp
@@ -0,0 +1,93 @@
+#pragma once
+#include <string>
+#include <vector>
+#include <memory>
+#include <expected>
+#include <cstdint>
+#include "pdfengine/pdf_document.hpp"
+
+namespace pdfengine {
+
+struct Rect {
+    double x = 0.0;
+    double y = 0.0;
+    double width = 0.0;
+    double height = 0.0;
+};
+
+using RectList = std::vector<Rect>;
+
+struct ParagraphBounds {
+    Rect rect;
+};
+
+struct GlyphInfo {
+    uint32_t glyphId = 0;
+    uint32_t cluster = 0;
+    double x = 0.0;
+    double y = 0.0;
+    double advance = 0.0;
+    double width = 0.0;
+    double ascent = 0.0;
+    double descent = 0.0;
+    std::string text;
+};
+
+struct LineInfo {
+    int id = 0;
+    Rect rect;
+    double baselineY = 0.0;
+};
+
+struct CaretState {
+    int offset = 0;
+    Rect rect;
+};
+
+// Internal comprehensive layout state
+struct LayoutResult {
+    ParagraphBounds bounds;
+    std::vector<LineInfo> lines;
+    std::vector<GlyphInfo> glyphs;
+    std::vector<Rect> selectionRects;
+    CaretState caret;
+    RectList dirtyRects;
+};
+
+// Stable, lightweight view for WASM export
+struct LayoutView {
+    std::vector<LineInfo> lines;
+    std::vector<GlyphInfo> glyphs;
+    CaretState caret;
+    RectList dirtyRects;
+};
+
+class EditSession {
+public:
+    virtual ~EditSession() = default;
+
+    static std::shared_ptr<EditSession> StartEditSession(
+        std::shared_ptr<PdfDocument> doc, 
+        int pageIndex, 
+        const std::string& paraId
+    );
+
+    // Returns a stable, lightweight view of the layout
+    virtual LayoutView GetLayoutView() const = 0;
+    
+    // Geometry queries against the cached layout
+    // offset represents the caret insertion point (between glyphs)
+    virtual int HitTest(double x, double y) const = 0;
+    virtual Rect GetCaretRect(int offset) const = 0;
+    virtual std::vector<Rect> GetSelectionRects(int startOffset, int endOffset) const = 0;
+
+    // Mutates paragraph, marks cache dirty, recalculates
+    virtual void ApplyEdit(const std::string& editOpJson) = 0;
+    
+    // Rendering & Lifecycle
+    virtual std::vector<uint8_t> RenderDirtyRegion(int dpi, const Rect& region) const = 0;
+    virtual bool CommitEdit() = 0;
+    virtual void CancelEdit() = 0;
+};
+
+} // namespace pdfengine
diff --git a/engine/include/pdfengine/pdf_document.hpp b/engine/include/pdfengine/pdf_document.hpp
index 9b668b9..866eed8 100644
--- a/engine/include/pdfengine/pdf_document.hpp
+++ b/engine/include/pdfengine/pdf_document.hpp
@@ -1,12 +1,21 @@
 #pragma once
 
-#include <expected>
 #include <memory>
 #include <string>
 #include <vector>
 #include <cstdint>
 #include <array>
 
+#if __has_include(<expected>)
+#include <expected>
+#elif __has_include(<tl/expected.hpp>)
+#include <tl/expected.hpp>
+namespace std {
+using tl::expected;
+using tl::unexpected;
+}
+#endif
+
 namespace pdfengine {
 
 enum class EngineError {
@@ -276,6 +285,7 @@ public:
     virtual std::expected<std::vector<InvalidatedRegion>, EngineError> applyEdits(const std::string& editsJson) = 0;
 
 
+    [[nodiscard]] virtual std::string validateLayout(int pageIndex, const std::string& jsonStr) { (void)pageIndex; (void)jsonStr; return "{}"; }
     [[nodiscard]] virtual std::string lastReflowLayout() const { return {}; }
 
     [[nodiscard]] virtual bool lastReflowOverflowed() const { return false; }
diff --git a/engine/include/pdfengine/text_layout_engine.hpp b/engine/include/pdfengine/text_layout_engine.hpp
new file mode 100644
index 0000000..5f3bea4
--- /dev/null
+++ b/engine/include/pdfengine/text_layout_engine.hpp
@@ -0,0 +1,66 @@
+#pragma once
+
+#include "pdfengine/edit_session.hpp"
+#include "fonts/shaping/hb_shaper.hpp"
+#include "fonts/face/font_face.hpp"
+#include <string>
+#include <vector>
+#include <memory>
+
+namespace pdfengine::text {
+
+struct LayoutConstraints {
+    double columnLeft = 0.0;
+    double columnRight = 0.0;
+    double firstBaselineY = 0.0;
+    double leading = 0.0;
+    std::string align = "left";
+    double hangingIndent = 0.0;
+};
+
+class LineBreaker {
+public:
+    LineBreaker(const LayoutConstraints& constraints);
+    
+    // Processes a stream of shaped glyphs and applies line breaking
+    void ProcessRun(
+        const std::vector<fonts::ShapedGlyph>& shapedGlyphs,
+        const std::string& runText,
+        double fontSize,
+        const fonts::FontFace& face,
+        double scale
+    );
+
+    // Finalizes the layout and populates the LayoutResult
+    void Finalize(LayoutResult& outLayout);
+
+private:
+    LayoutConstraints constraints_;
+    double currentX_;
+    double currentY_;
+    
+    // Internal state for word wrapping
+    std::vector<GlyphInfo> currentLineGlyphs_;
+    std::vector<LineInfo> finishedLines_;
+    std::vector<GlyphInfo> allGlyphs_;
+
+    void CommitLine();
+};
+
+class TextLayoutEngine {
+public:
+    TextLayoutEngine();
+    
+    // Computes layout for a given text run (simplified for single font for now)
+    LayoutResult ComputeLayout(
+        const std::string& text,
+        const LayoutConstraints& constraints,
+        fonts::FontFace& fontFace,
+        double fontSize
+    );
+
+private:
+    fonts::HbShaper shaper_;
+};
+
+} // namespace pdfengine::text
diff --git a/engine/src/parser/pdfium_document.hpp b/engine/src/parser/pdfium_document.hpp
index d710919..a4914e0 100644
--- a/engine/src/parser/pdfium_document.hpp
+++ b/engine/src/parser/pdfium_document.hpp
@@ -103,6 +103,7 @@ public:
     void invalidateCaches();
 
     std::expected<std::vector<InvalidatedRegion>, EngineError> applyEdits(const std::string& editsJson) override;
+    std::string validateLayout(int pageIndex, const std::string& jsonStr) override;
     std::expected<std::vector<uint8_t>, EngineError> saveIncremental() const override;
     std::expected<std::vector<uint8_t>, EngineError> saveFull() const override;
     std::expected<std::vector<uint8_t>, EngineError> saveFullForExport() const override;
@@ -151,13 +152,14 @@ private:
                                   double fontSize, const std::vector<uint32_t>& codepoints,
                                   const std::vector<int>& srcObjects = {},
                                   FPDF_FONT reuseFont = nullptr);
+#endif
 
     mutable std::unordered_map<std::string, fonts::pdf_fonts::ReconstructedFont> reconstructedFonts_;
     mutable std::mutex reconstructedFontsMutex_;
     void registerAuxFont(const std::string& internalFontId, const std::vector<uint8_t>& sfnt) override;
     std::expected<std::vector<uint8_t>, EngineError> getReconstructedFontData(const std::string& internalFontId) override;
     const fonts::pdf_fonts::ReconstructedFont* lookupReconFont(const std::string& internalFontId);
-#ifdef PDFENGINE_WITH_QPDF
+#if defined(PDFENGINE_WITH_PDFIUM) && defined(PDFENGINE_WITH_QPDF)
     const fonts::pdf_fonts::ReconstructedFont* getReconstructedEmbeddedFont(const std::string& internalFontId);
 #endif
 
@@ -180,7 +182,6 @@ private:
     std::expected<void, EngineError> applyOp_pageReorder(const nlohmann::json& op, int pageIndex);
     std::expected<void, EngineError> applyOp_deleteAnnotation(const nlohmann::json& op, int pageIndex);
     std::expected<void, EngineError> applyOp_updateAnnotation(const nlohmann::json& op, int pageIndex);
-#endif
 };
 
 std::vector<unsigned short> utf8_to_utf16le(const std::string& utf8);
diff --git a/engine/src/parser/pdfium_edit_pages.cpp b/engine/src/parser/pdfium_edit_pages.cpp
index 0e255b1..7b6fe8a 100644
--- a/engine/src/parser/pdfium_edit_pages.cpp
+++ b/engine/src/parser/pdfium_edit_pages.cpp
@@ -133,19 +133,23 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_pageRotation(const nloh
 #endif
 }
 
-#ifdef PDFENGINE_WITH_PDFIUM
 std::expected<void, EngineError> PdfiumDocument::applyOp_pageDeletion(const nlohmann::json& op, int pageIndex) {
+#ifdef PDFENGINE_WITH_PDFIUM
+                (void)op;
                 if (pageCount() <= 1) {
                     spdlog::error("Cannot delete the only page in the document");
                     return std::unexpected(EngineError::Unknown);
                 }
                 FPDFPage_Delete(doc_, pageIndex);
     return {};
-}
+#else
+    (void)op; (void)pageIndex;
+    return std::unexpected(EngineError::Unknown);
 #endif
+}
 
-#ifdef PDFENGINE_WITH_PDFIUM
 std::expected<void, EngineError> PdfiumDocument::applyOp_pageReorder(const nlohmann::json& op, int pageIndex) {
+#ifdef PDFENGINE_WITH_PDFIUM
                 if (!op.contains("data") || !op["data"].is_object()) {
                     spdlog::error("page_reorder operation missing 'data' object");
                     return std::unexpected(EngineError::InvalidFormat);
@@ -167,7 +171,10 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_pageReorder(const nlohm
                     return std::unexpected(EngineError::Unknown);
                 }
     return {};
-}
+#else
+    (void)op; (void)pageIndex;
+    return std::unexpected(EngineError::Unknown);
 #endif
+}
 
 }
\ No newline at end of file
diff --git a/engine/src/parser/pdfium_edit_reflow.cpp b/engine/src/parser/pdfium_edit_reflow.cpp
index 5dc3ea5..acf807a 100644
--- a/engine/src/parser/pdfium_edit_reflow.cpp
+++ b/engine/src/parser/pdfium_edit_reflow.cpp
@@ -1,5 +1,7 @@
 #include "parser/pdfium_internal.hpp"
-
+#include "fonts/face/free_type_manager.hpp"
+#include "fonts/face/font_face.hpp"
+#include "pdfengine/text_layout_engine.hpp"
 namespace pdfengine::parser {
 
 std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::json& op, int pageIndex) {
@@ -30,7 +32,7 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
                     RunStyle rs;
                     rs.text = rj.value("text", "");
                     rs.internalFontId = rj.value("internalFontId", "");
-                    rs.fontSize = rj.value("fontSize", 12.0);
+                    rs.fontSize = rj.value("fontSize", 0.0);
                     parseHex(rj.value("color", std::string("#000000")), rs.r, rs.g, rs.b);
                     if (rj.contains("advances") && rj["advances"].is_array()) {
                         for (const auto& a : rj["advances"]) rs.advances.push_back(a.get<double>());
@@ -142,46 +144,43 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
                         spdlog::debug("reflow_paragraph: adopted {} anchor-page object(s) by paraId", adoptedById);
                 }
 
-                {
-                    std::unordered_map<std::string, double> exactByBaseName;
-                    std::vector<double> exactSizes;
-                    for (int idx : paragraphSet) {
-                        FPDF_PAGEOBJECT o = FPDFPage_GetObject(page, idx);
-                        if (!o || FPDFPageObj_GetType(o) != FPDF_PAGEOBJ_TEXT) continue;
+                double exactNominal = 0.0;
+                double exactScaleX = 1.0;
+                double exactScaleY = 1.0;
+                for (int idx : paragraphSet) {
+                    FPDF_PAGEOBJECT o = FPDFPage_GetObject(page, idx);
+                    if (!o || FPDFPageObj_GetType(o) != FPDF_PAGEOBJ_TEXT) continue;
+                    float nom = 0;
+                    if (FPDFTextObj_GetFontSize(o, &nom) && nom > 0.1) {
                         FS_MATRIX mtx;
-                        if (!FPDFPageObj_GetMatrix(o, &mtx)) continue;
-                        float nominal = 0.0f;
-                        if (!FPDFTextObj_GetFontSize(o, &nominal)) continue;
-                        double scale = std::sqrt(static_cast<double>(mtx.a) * mtx.a +
-                                                 static_cast<double>(mtx.b) * mtx.b);
-                        double exact = static_cast<double>(nominal) * scale;
-                        if (exact <= 0.1) continue;
-                        exactSizes.push_back(exact);
-                        FPDF_FONT fo = FPDFTextObj_GetFont(o);
-                        if (!fo) continue;
-                        size_t nl = FPDFFont_GetBaseFontName(fo, nullptr, 0);
-                        if (nl == 0) continue;
-                        std::vector<char> nb(nl);
-                        if (FPDFFont_GetBaseFontName(fo, nb.data(), nl) == 0) continue;
-                        std::string bn(nb.data());
-                        if (!exactByBaseName.count(bn)) exactByBaseName[bn] = exact;
-                    }
-                    double paraExact = 0.0;
-                    if (!exactSizes.empty()) {
-                        std::sort(exactSizes.begin(), exactSizes.end());
-                        paraExact = exactSizes[exactSizes.size() / 2]; 
-                    }
-                    if (paraExact > 0.1) {
-                        for (auto& rs : runs) {
-                            std::string bn = baseNameFromInternalFontId(rs.internalFontId);
-                            auto it = exactByBaseName.find(bn);
-                            rs.fontSize = (it != exactByBaseName.end() && it->second > 0.1) ? it->second : paraExact;
+                        if (FPDFPageObj_GetMatrix(o, &mtx)) {
+                            exactScaleX = std::sqrt(static_cast<double>(mtx.a) * mtx.a + static_cast<double>(mtx.b) * mtx.b);
+                            exactScaleY = std::sqrt(static_cast<double>(mtx.c) * mtx.c + static_cast<double>(mtx.d) * mtx.d);
+                            exactNominal = nom;
+                            break;
                         }
-                        spdlog::info("reflow_paragraph: size-exact override paraExact={:.2f} ({} font(s))",
-                                     paraExact, exactByBaseName.size());
                     }
                 }
 
+                double textAspect = 1.0;
+                if (exactNominal > 0.1 && exactScaleY > 0.001) {
+                    double trueVerticalSize = exactNominal * exactScaleY;
+                    for (auto& rs : runs) {
+                        spdlog::info("[STAGE_3_WASM_INPUT] fontSize={:.2f}, trueVerticalSize={:.2f}, exactNominal={:.2f}, exactScaleY={:.4f}", rs.fontSize, trueVerticalSize, exactNominal, exactScaleY);
+                        if (rs.fontSize <= 0.0 || rs.fontSize < trueVerticalSize * 0.85) {
+                            rs.fontSize = trueVerticalSize;
+                        }
+                    }
+                    textAspect = exactScaleX / exactScaleY;
+                } else {
+                    for (auto& rs : runs) {
+                        if (rs.fontSize <= 0.0) {
+                            rs.fontSize = 12.0;
+                        }
+                    }
+                }
+                spdlog::info("[FONT_METRICS_DEBUG] exactNominal={:.2f}, exactScaleX={:.2f}, exactScaleY={:.2f}, trueVerticalSize={:.2f}, textAspect={:.2f}", exactNominal, exactScaleX, exactScaleY, exactNominal * exactScaleY, textAspect);
+
                 std::vector<EmissionFont> runFonts(runs.size());
                 auto toCodepoints = [](const std::string& s) {
                     auto u16 = utf8_to_utf16le(s);
@@ -241,7 +240,7 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
                     if (text.empty()) return out;
                     auto& rf = runFonts[runIdx];
                     double size = runs[runIdx].fontSize > 0 ? runs[runIdx].fontSize : 12.0;
-                    double scale = size / static_cast<double>(kRefSize);
+                    double scale = (size * textAspect) / static_cast<double>(kRefSize);
                     fonts::FontFace* face = rf.measureFace ? rf.measureFace.get()
                                           : (rf.resolved ? &rf.resolved->getFontFace() : nullptr);
                     if (face) {
@@ -259,6 +258,9 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
                     return out;
                 };
 
+                // Forensic flag: force whole-run emission while keeping client advances.
+                // Set data.forensicForceWholeRun=true to isolate whether runPerChar causes the visual change.
+                const bool forensicForceWholeRun = data.value("forensicForceWholeRun", false);
                 std::vector<std::vector<double>> runCharAdv(runs.size());
                 std::vector<char> runPerChar(runs.size(), 0);
                 for (size_t ri = 0; ri < runs.size(); ++ri) {
@@ -268,9 +270,17 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
                         bool diverges = natural.size() != runCharAdv[ri].size();
                         for (size_t c = 0; !diverges && c < natural.size(); ++c)
                             if (std::abs(natural[c] - runCharAdv[ri][c]) > 0.05) diverges = true;
-                        runPerChar[ri] = diverges ? 1 : 0;
+                        runPerChar[ri] = (forensicForceWholeRun ? 0 : (diverges ? 1 : 0));
+                        spdlog::info("[FORENSIC_RUNPERCHAR] run={} textLen={} advLen={} diverges={} forceWhole={} runPerChar={} natural0={:.4f} client0={:.4f} measureFace={}",
+                            ri, runs[ri].text.size(), runs[ri].advances.size(), diverges,
+                            forensicForceWholeRun, (int)runPerChar[ri],
+                            natural.empty() ? -1.0 : natural[0],
+                            runCharAdv[ri].empty() ? -1.0 : runCharAdv[ri][0],
+                            (bool)(runFonts[ri].measureFace));
                     } else {
                         runCharAdv[ri] = perCharAdvances(ri, runs[ri].text);
+                        spdlog::info("[FORENSIC_RUNPERCHAR] run={} textLen={} advLen={} -> recomputed advances (no client match) runPerChar=0 forceWhole={}",
+                            ri, runs[ri].text.size(), runs[ri].advances.size(), forensicForceWholeRun);
                     }
                 }
                 auto charAdvAt = [&](int ri, size_t off) -> double {
@@ -336,6 +346,8 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
                         lineCont.push_back(firstLineOfSeg ? 0 : 1);
                         lineSegEnd.push_back(segEnd);
                     };
+                    spdlog::info("[KEYSTROKE_BACKEND_DEBUG] columnLeft={:.2f}, columnRight={:.2f}, columnWidth={:.2f}, firstBaselineY={:.2f}, oldLineCount={}, hangingIndent={:.2f}",
+                                 columnLeft, columnRight, columnWidth, firstBaselineY, oldLineCount, hangingIndent);
                     for (size_t k = 0; k < allWords.size(); ++k) {
                         size_t wi = allWords[k];
                         if (wi == kHardBreak) {       // forced break: end the current line (may be blank)
@@ -346,6 +358,8 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
                         double effW = columnWidth - (firstLineOfSeg ? 0.0 : hangingIndent);
                         double gap = (cur.empty() || prevWord == kHardBreak) ? 0.0 : words[prevWord].spaceAfter;
                         if (!cur.empty() && curW + gap + words[wi].width > effW) {
+                            spdlog::info("[KEYSTROKE_BACKEND_WRAP] Wrapped at word index {} ('{}'): curW={:.2f}, gap={:.2f}, wordW={:.2f}, sum={:.2f} > effW={:.2f} (columnWidth={:.2f})",
+                                         wi, words[wi].segs.empty() ? "" : words[wi].segs[0].text, curW, gap, words[wi].width, curW + gap + words[wi].width, effW, columnWidth);
                             pushLine(0);              
                             cur.clear(); firstLineOfSeg = false;
                             cur.push_back(wi); curW = words[wi].width;
@@ -421,20 +435,71 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
                             lineText.push_back(' ');
                             adv.push_back(gap);
                         }
-                        double segX = x;
                         for (auto& seg : words[wi].segs) {
-                            if (lineFontSize <= 0.0) lineFontSize = runs[seg.runIdx].fontSize;
+                            double segX = x;
                             FPDF_FONT font = runFonts[seg.runIdx].font;
+                            double emitFontSize = runs[seg.runIdx].fontSize;
+                            double mtxScaleX = textAspect;
+                            double mtxScaleY = 1.0;
+                            if (exactNominal > 0.1 && exactScaleY > 0.001) {
+                                emitFontSize = exactNominal;
+                                mtxScaleX = exactScaleX;
+                                mtxScaleY = exactScaleY;
+                                lineFontSize = exactNominal * exactScaleY;
+                            } else {
+                                lineFontSize = (std::max)(lineFontSize, runs[seg.runIdx].fontSize);
+                            }
+                            spdlog::info("[STAGE_4_REFLOW_OUTPUT] lineFontSize={:.2f}, emitFontSize={:.2f}, mtxScaleY={:.4f}", lineFontSize, emitFontSize, mtxScaleY);
                             auto emitObj = [&](const std::string& s, double atX) {
                                 if (!font || s.empty()) return;
-                                FPDF_PAGEOBJECT obj = FPDFPageObj_CreateTextObj(doc_, font, static_cast<float>(runs[seg.runIdx].fontSize));
+                                std::string baseFontName;
+                                {
+                                    size_t nl = FPDFFont_GetBaseFontName(font, nullptr, 0);
+                                    if (nl > 0) {
+                                        std::vector<char> nb(nl);
+                                        if (FPDFFont_GetBaseFontName(font, nb.data(), nl) > 0)
+                                            baseFontName = nb.data();
+                                    }
+                                }
+                                const auto& ef = runFonts[seg.runIdx];
+                                spdlog::info("[EMIT_FONT] text='{}' baseFont='{}' fontPtr={} measureFacePtr={} "
+                                             "hasResolved={} fontSize={:.4f} atX={:.4f} baselineY={:.4f} "
+                                             "mtxScaleX={:.4f} mtxScaleY={:.4f} runPerChar={} internalFontId='{}'",
+                                    s, baseFontName, (void*)font,
+                                    (void*)(ef.measureFace ? ef.measureFace.get() : nullptr),
+                                    (bool)ef.resolved, emitFontSize, atX, baselineY,
+                                    mtxScaleX, mtxScaleY, (int)runPerChar[seg.runIdx],
+                                    runs[seg.runIdx].internalFontId);
+                                FPDF_PAGEOBJECT obj = FPDFPageObj_CreateTextObj(doc_, font, static_cast<float>(emitFontSize));
                                 if (!obj) return;
                                 FPDFPageObj_SetFillColor(obj, runs[seg.runIdx].r, runs[seg.runIdx].g, runs[seg.runIdx].b, 255);
                                 auto u16 = utf8_to_utf16le(s); u16.push_back(0);
                                 FPDFText_SetText(obj, reinterpret_cast<FPDF_WIDESTRING>(u16.data()));
-                                FPDFPageObj_Transform(obj, 1.0, 0.0, 0.0, 1.0, atX, baselineY);
+                                FPDFPageObj_Transform(obj, mtxScaleX, 0.0, 0.0, mtxScaleY, atX, baselineY);
                                 repagSetParaId(doc_, obj, paraId);
                                 FPDFPage_InsertObjectAtIndex(page, obj, minIndex);
+                                FS_MATRIX objMtx;
+                                FPDFPageObj_GetMatrix(obj, &objMtx);
+                                float objFS = 0;
+                                FPDFTextObj_GetFontSize(obj, &objFS);
+                                float l = 0, b = 0, r = 0, t = 0;
+                                FPDFPageObj_GetBounds(obj, &l, &b, &r, &t);
+                                FPDF_FONT objFont = FPDFTextObj_GetFont(obj);
+                                std::string objBase;
+                                if (objFont) {
+                                    size_t nl = FPDFFont_GetBaseFontName(objFont, nullptr, 0);
+                                    if (nl > 0) {
+                                        std::vector<char> nb(nl);
+                                        if (FPDFFont_GetBaseFontName(objFont, nb.data(), nl) > 0)
+                                            objBase = nb.data();
+                                    }
+                                }
+                                spdlog::info("[EDITED_TEXT_OBJECT_DEBUG] text='{}' objBaseFont='{}' fontSize={:.2f}, "
+                                             "matrix=[{:.4f}, {:.4f}, {:.4f}, {:.4f}, {:.4f}, {:.4f}], "
+                                             "bbox=[{:.2f}, {:.2f}, {:.2f}, {:.2f}]",
+                                             s, objBase, objFS,
+                                             objMtx.a, objMtx.b, objMtx.c, objMtx.d, objMtx.e, objMtx.f,
+                                             l, b, r, t);
                             };
                             if (runPerChar[seg.runIdx]) {
                                 double gx = segX;
@@ -475,6 +540,7 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
                 }
                 lastReflowLayout_ = nlohmann::json{
                     {"columnLeft", columnLeft}, {"anchorPage", pageIndex}, {"lines", layoutLines}}.dump();
+                spdlog::info("[STAGE_5_SERIALIZED_JSON] {}", lastReflowLayout_);
 
                 if (!FPDFPage_GenerateContent(page)) {
                     spdlog::error("Failed to generate page content after reflow_paragraph");
@@ -500,4 +566,61 @@ std::expected<void, EngineError> PdfiumDocument::applyOp_reflow(const nlohmann::
 #endif
 }
 
+std::string PdfiumDocument::validateLayout(int pageIndex, const std::string& jsonStr) {
+#ifdef PDFENGINE_WITH_PDFIUM
+    try {
+        auto data = nlohmann::json::parse(jsonStr);
+        if (!data.is_object()) return "{}";
+        
+        std::string text = "";
+        std::string internalFontId = "";
+        double fontSize = 12.0;
+        
+        if (data.contains("runs") && data["runs"].is_array() && !data["runs"].empty()) {
+            auto run = data["runs"][0];
+            text = run.value("text", "");
+            internalFontId = run.value("internalFontId", "");
+            fontSize = run.value("fontSize", 12.0);
+        }
+        
+        text::LayoutConstraints constraints;
+        constraints.columnLeft = data.value("columnLeft", 0.0);
+        constraints.columnRight = data.value("columnRight", 0.0);
+        constraints.firstBaselineY = data.value("firstBaselineY", 0.0);
+        constraints.leading = data.value("leading", 0.0);
+        
+        auto fontDataRes = getFontData(internalFontId);
+        auto face = std::make_shared<fonts::FontFace>();
+        if (!fontDataRes || !face->loadFromMemory(*fontDataRes)) {
+            return "{}";
+        }
+        
+        text::TextLayoutEngine engine;
+        auto layout = engine.ComputeLayout(text, constraints, *face, fontSize);
+        
+        nlohmann::json out;
+        out["lines"] = nlohmann::json::array();
+        for (const auto& l : layout.lines) {
+            out["lines"].push_back({
+                {"rect", {{"x", l.rect.x}, {"y", l.rect.y}, {"width", l.rect.width}, {"height", l.rect.height}}}
+            });
+        }
+        out["glyphs"] = nlohmann::json::array();
+        for (const auto& g : layout.glyphs) {
+            out["glyphs"].push_back({
+                {"x", g.x}, {"y", g.y}, {"width", g.width}, {"advance", g.advance},
+                {"ascent", g.ascent}, {"descent", g.descent}, {"cluster", g.cluster},
+                {"text", g.text}
+            });
+        }
+        return out.dump();
+    } catch (...) {
+        return "{}";
+    }
+#else
+    (void)pageIndex; (void)jsonStr;
+    return "{}";
+#endif
+}
+
 } 
\ No newline at end of file
diff --git a/engine/src/parser/pdfium_edit_session.cpp b/engine/src/parser/pdfium_edit_session.cpp
new file mode 100644
index 0000000..88f2b00
--- /dev/null
+++ b/engine/src/parser/pdfium_edit_session.cpp
@@ -0,0 +1,180 @@
+#include "pdfengine/edit_session.hpp"
+#include "parser/pdfium_edit_session.hpp"
+#include "parser/pdfium_document.hpp"
+#include <spdlog/spdlog.h>
+#include <cmath>
+
+namespace pdfengine {
+
+std::shared_ptr<EditSession> EditSession::StartEditSession(
+    std::shared_ptr<PdfDocument> doc, 
+    int pageIndex, 
+    const std::string& paraId
+) {
+    if (!doc) {
+        spdlog::error("StartEditSession: null document");
+        return nullptr;
+    }
+    return std::make_shared<parser::PdfiumEditSession>(doc, pageIndex, paraId);
+}
+
+} // namespace pdfengine
+
+namespace pdfengine::parser {
+
+PdfiumEditSession::PdfiumEditSession(std::shared_ptr<PdfDocument> doc, int pageIndex, const std::string& paraId)
+    : doc_(doc), pageIndex_(pageIndex), paraId_(paraId) {
+    spdlog::info("Started edit session on page {} for para {}", pageIndex_, paraId_);
+    
+    // In a full implementation, we'd extract text, bounds, and fonts from doc_ using paraId_.
+    // For now, populate dummy state for unit testing the architecture.
+    currentText_ = "Mock initial text for paragraph.";
+    constraints_.columnLeft = 100.0;
+    constraints_.columnRight = 400.0;
+    constraints_.firstBaselineY = 700.0;
+    constraints_.leading = 14.0;
+    isDirty_ = true;
+}
+
+PdfiumEditSession::~PdfiumEditSession() {}
+
+void PdfiumEditSession::RebuildLayoutIfNeeded() const {
+    if (!isDirty_) return;
+
+    if (currentFace_) {
+        cachedLayout_ = layoutEngine_.ComputeLayout(
+            currentText_, constraints_, *currentFace_, currentFontSize_
+        );
+    } else {
+        // Mock fallback if face not loaded yet
+        cachedLayout_.lines.clear();
+        cachedLayout_.glyphs.clear();
+        
+        GlyphInfo g;
+        g.x = constraints_.columnLeft;
+        g.y = constraints_.firstBaselineY;
+        g.width = 5.0;
+        g.advance = 6.0;
+        g.ascent = 10.0;
+        g.descent = -2.0;
+        g.cluster = 0;
+        
+        if (!currentText_.empty()) {
+            LineInfo line;
+            line.id = 0;
+            line.baselineY = constraints_.firstBaselineY;
+            
+            for (size_t i = 0; i < currentText_.size(); ++i) {
+                g.cluster = static_cast<uint32_t>(i);
+                g.text = currentText_.substr(i, 1);
+                cachedLayout_.glyphs.push_back(g);
+                g.x += g.advance;
+            }
+            
+            line.rect = Rect{constraints_.columnLeft, constraints_.firstBaselineY - g.ascent, 
+                             g.x - constraints_.columnLeft, g.ascent - g.descent};
+            cachedLayout_.lines.push_back(line);
+        }
+    }
+    
+    isDirty_ = false;
+}
+
+LayoutView PdfiumEditSession::GetLayoutView() const {
+    RebuildLayoutIfNeeded();
+    return LayoutView{
+        cachedLayout_.lines,
+        cachedLayout_.glyphs,
+        cachedLayout_.caret,
+        cachedLayout_.dirtyRects
+    };
+}
+
+int PdfiumEditSession::HitTest(double x, double y) const {
+    RebuildLayoutIfNeeded();
+    
+    // Find closest glyph by checking boundary/midpoint logic
+    int closestCluster = 0;
+    double minDistanceSq = 1e9;
+    
+    for (const auto& glyph : cachedLayout_.glyphs) {
+        // Find distance to bounding box
+        double dx = 0.0;
+        if (x < glyph.x) dx = glyph.x - x;
+        else if (x > glyph.x + glyph.advance) dx = x - (glyph.x + glyph.advance);
+        
+        double gy = glyph.y - glyph.ascent;
+        double gh = glyph.ascent - glyph.descent;
+        double dy = 0.0;
+        if (y < gy) dy = gy - y;
+        else if (y > gy + gh) dy = y - (gy + gh);
+        
+        double distSq = dx * dx + dy * dy;
+        if (distSq < minDistanceSq) {
+            minDistanceSq = distSq;
+            // Midpoint logic: if past midpoint, insertion point is after this glyph
+            if (x > glyph.x + glyph.advance / 2.0) {
+                closestCluster = glyph.cluster + 1;
+            } else {
+                closestCluster = glyph.cluster;
+            }
+        }
+    }
+    
+    return closestCluster;
+}
+
+Rect PdfiumEditSession::GetCaretRect(int offset) const {
+    RebuildLayoutIfNeeded();
+    
+    if (cachedLayout_.glyphs.empty()) {
+        return Rect{constraints_.columnLeft, constraints_.firstBaselineY - 10.0, 1.0, 12.0};
+    }
+    
+    // Caret is positioned *between* glyphs at the insertion point `offset`.
+    // If offset matches a cluster, place it at the left edge of that glyph.
+    for (const auto& glyph : cachedLayout_.glyphs) {
+        if (glyph.cluster == static_cast<uint32_t>(offset)) {
+            return Rect{glyph.x, glyph.y - glyph.ascent, 1.0, glyph.ascent - glyph.descent};
+        }
+    }
+    
+    // If offset is beyond all glyphs, place it at the right edge of the last glyph.
+    const auto& lastGlyph = cachedLayout_.glyphs.back();
+    return Rect{lastGlyph.x + lastGlyph.advance, lastGlyph.y - lastGlyph.ascent, 1.0, lastGlyph.ascent - lastGlyph.descent};
+}
+
+std::vector<Rect> PdfiumEditSession::GetSelectionRects(int startOffset, int endOffset) const {
+    RebuildLayoutIfNeeded();
+    (void)startOffset; (void)endOffset;
+    return {};
+}
+
+void PdfiumEditSession::ApplyEdit(const std::string& editOpJson) {
+    // Phase 1.1: Modify state and mark dirty
+    spdlog::info("ApplyEdit called with {}", editOpJson);
+    
+    // Very naive edit implementation for testing
+    // e.g. editOpJson = "append:A"
+    if (editOpJson.find("append:") == 0) {
+        currentText_ += editOpJson.substr(7);
+    }
+    
+    isDirty_ = true;
+}
+
+std::vector<uint8_t> PdfiumEditSession::RenderDirtyRegion(int dpi, const Rect& region) const {
+    (void)dpi; (void)region;
+    return {};
+}
+
+bool PdfiumEditSession::CommitEdit() {
+    spdlog::info("CommitEdit called");
+    return true;
+}
+
+void PdfiumEditSession::CancelEdit() {
+    spdlog::info("CancelEdit called");
+}
+
+} // namespace pdfengine::parser
diff --git a/engine/src/parser/pdfium_edit_session.hpp b/engine/src/parser/pdfium_edit_session.hpp
new file mode 100644
index 0000000..8bb8536
--- /dev/null
+++ b/engine/src/parser/pdfium_edit_session.hpp
@@ -0,0 +1,46 @@
+#pragma once
+
+#include "pdfengine/edit_session.hpp"
+#include "pdfengine/text_layout_engine.hpp"
+#include "parser/pdfium_internal.hpp"
+#include <memory>
+#include <string>
+
+namespace pdfengine::parser {
+
+class PdfiumEditSession : public EditSession {
+public:
+    PdfiumEditSession(std::shared_ptr<PdfDocument> doc, int pageIndex, const std::string& paraId);
+    ~PdfiumEditSession() override;
+
+    LayoutView GetLayoutView() const override;
+    int HitTest(double x, double y) const override;
+    Rect GetCaretRect(int offset) const override;
+    std::vector<Rect> GetSelectionRects(int startOffset, int endOffset) const override;
+
+    void ApplyEdit(const std::string& editOpJson) override;
+    
+    std::vector<uint8_t> RenderDirtyRegion(int dpi, const Rect& region) const override;
+    bool CommitEdit() override;
+    void CancelEdit() override;
+
+private:
+    void RebuildLayoutIfNeeded() const;
+
+    std::shared_ptr<PdfDocument> doc_;
+    int pageIndex_;
+    std::string paraId_;
+
+    // Live editing state
+    std::string currentText_;
+    text::LayoutConstraints constraints_;
+    std::shared_ptr<fonts::FontFace> currentFace_;
+    double currentFontSize_ = 12.0;
+
+    // Layout cache and engine
+    mutable text::TextLayoutEngine layoutEngine_;
+    mutable LayoutResult cachedLayout_;
+    mutable bool isDirty_ = true;
+};
+
+} // namespace pdfengine::parser
diff --git a/engine/src/parser/pdfium_fonts.cpp b/engine/src/parser/pdfium_fonts.cpp
index 785ede4..8a1c583 100644
--- a/engine/src/parser/pdfium_fonts.cpp
+++ b/engine/src/parser/pdfium_fonts.cpp
@@ -2,8 +2,8 @@
 
 namespace pdfengine::parser {
 
-#ifdef PDFENGINE_WITH_PDFIUM
 void PdfiumDocument::registerAuxFont(const std::string& internalFontId, const std::vector<uint8_t>& sfnt) {
+#ifdef PDFENGINE_WITH_PDFIUM
     fonts::pdf_fonts::ReconstructedFont rf;
     if (!sfnt.empty()) {
         rf.ok = true;
@@ -14,16 +14,25 @@ void PdfiumDocument::registerAuxFont(const std::string& internalFontId, const st
     }
     std::lock_guard<std::mutex> lock(reconstructedFontsMutex_);
     reconstructedFonts_[internalFontId] = std::move(rf);
+#else
+    (void)internalFontId; (void)sfnt;
+#endif
 }
 
 std::expected<std::vector<uint8_t>, EngineError>
 PdfiumDocument::getReconstructedFontData(const std::string& internalFontId) {
+#ifdef PDFENGINE_WITH_PDFIUM
     const auto* rf = lookupReconFont(internalFontId);
     if (rf && rf->ok && !rf->sfnt.empty()) return rf->sfnt;
     return std::unexpected(EngineError::Unknown);
+#else
+    (void)internalFontId;
+    return std::unexpected(EngineError::Unknown);
+#endif
 }
 
 const fonts::pdf_fonts::ReconstructedFont* PdfiumDocument::lookupReconFont(const std::string& internalFontId) {
+#ifdef PDFENGINE_WITH_PDFIUM
 #ifdef PDFENGINE_WITH_QPDF
     return getReconstructedEmbeddedFont(internalFontId);
 #else
@@ -31,8 +40,11 @@ const fonts::pdf_fonts::ReconstructedFont* PdfiumDocument::lookupReconFont(const
     auto it = reconstructedFonts_.find(internalFontId);
     return it != reconstructedFonts_.end() ? &it->second : nullptr;
 #endif
-}
+#else
+    (void)internalFontId;
+    return nullptr;
 #endif
+}
 
 #if defined(PDFENGINE_WITH_PDFIUM) && defined(PDFENGINE_WITH_QPDF)
 const fonts::pdf_fonts::ReconstructedFont*
@@ -104,11 +116,20 @@ PdfiumDocument::EmissionFont PdfiumDocument::loadEmissionFont(
     if (out.resolved) {
         for (uint32_t cp : codepoints) {
             if (!out.resolved->hasGlyph(cp)) {
-                if (isSubsetFont) subsetLacksGlyphs = true; else fontSupportsAll = false;
+                if (isSubsetFont) {
+                    subsetLacksGlyphs = true;
+                    spdlog::info("[FONT_COVERAGE_DEBUG] internalFontId='{}' subset MISSING codepoint U+{:04X} (char='{}')",
+                        internalFontId, cp, (cp >= 0x20 && cp < 0x7F) ? std::string(1, (char)cp) : std::string("?"));
+                } else {
+                    fontSupportsAll = false;
+                    spdlog::info("[FONT_COVERAGE_DEBUG] internalFontId='{}' full font MISSING codepoint U+{:04X} (char='{}')",
+                        internalFontId, cp, (cp >= 0x20 && cp < 0x7F) ? std::string(1, (char)cp) : std::string("?"));
+                }
             }
         }
     } else {
         fontSupportsAll = false;
+        spdlog::info("[FONT_COVERAGE_DEBUG] internalFontId='{}' no resolved face - forcing system fallback", internalFontId);
     }
 
     std::string cacheKey;
@@ -178,7 +199,30 @@ PdfiumDocument::EmissionFont PdfiumDocument::loadEmissionFont(
                 out.font = FPDFText_LoadFont(doc_, bytes.data(), static_cast<uint32_t>(bytes.size()), FPDF_FONT_TRUETYPE, true);
             }
         }
+        // CRITICAL FIX: Even though we are emitting with a system font (because the PDF subset
+        // font does not contain the newly typed glyph), we must still use the ORIGINAL embedded
+        // subset font face for HarfBuzz shaping / advance measurement. This preserves the exact
+        // glyph advance widths, ascent, descent, and visual size of all original characters so
+        // the text does not visually shrink or change weight after the first keystroke.
+        if (matchedFontInfo && matchedFontInfo->isEmbedded) {
+            std::vector<uint8_t> origBytes;
+            if (auto perObj = getFontDataFromObjects(pageIndex, srcObjects, matchedFontInfo->internalFontId);
+                perObj.has_value() && !perObj->empty()) {
+                origBytes = std::move(*perObj);
+            } else if (auto fontDataRes = getFontData(matchedFontInfo->internalFontId);
+                       fontDataRes.has_value() && !fontDataRes.value().empty()) {
+                origBytes = std::move(fontDataRes.value());
+            }
+            if (!origBytes.empty()) {
+                auto mf = std::make_shared<fonts::FontFace>();
+                if (mf->loadFromMemory(origBytes)) {
+                    out.measureFace = mf;
+                    spdlog::info("[FONT_METRICS_FIX] internalFontId='{}' preserving original embedded face as measureFace despite system emission font - visual size preserved", internalFontId);
+                }
+            }
+        }
     }
+
     if (!out.font && matchedFontInfo && matchedFontInfo->isEmbedded &&
         classifyFontFidelity(*matchedFontInfo) != "exact") {
         const auto* rf = lookupReconFont(internalFontId);
@@ -211,7 +255,24 @@ PdfiumDocument::EmissionFont PdfiumDocument::loadEmissionFont(
 
     if (!out.font) {
         out.font = FPDFText_LoadStandardFont(doc_, fontName.c_str());
+        spdlog::info("[FONT_FALLBACK_DEBUG] internalFontId='{}' useEmbedded={} useSystem={} isSubsetFont={} subsetLacksGlyphs={} fontSupportsAll={} cacheKey='{}' fallbackStd='{}'",
+            internalFontId, useEmbedded, useSystem,
+            matchedFontInfo ? matchedFontInfo->isSubset : false,
+            (bool)(!out.resolved ? false : (isSubsetFont ? subsetLacksGlyphs : !fontSupportsAll)),
+            (bool)fontSupportsAll, cacheKey, fontName);
         if (!out.font) out.font = FPDFText_LoadStandardFont(doc_, "Helvetica");
+        // Standard fonts have no SFNT bytes → measureFace cannot be created from them.
+        spdlog::info("[MEASUREFACE_NULL] reason=standard_font_fallback internalFontId='{}' "
+                     "isEmbedded={} matched={} useEmbedded={} useSystem={} fontName='{}' "
+                     "fontPtr={} — no TTF/OTF bytes available for HarfBuzz measureFace",
+            internalFontId,
+            matchedFontInfo ? matchedFontInfo->isEmbedded : false,
+            (bool)matchedFontInfo, useEmbedded, useSystem, fontName, (void*)out.font);
+    } else {
+        spdlog::info("[FONT_LOAD_DEBUG] internalFontId='{}' useEmbedded={} useSystem={} isSubsetFont={} measureFace={} cacheKey='{}'",
+            internalFontId, useEmbedded, useSystem,
+            matchedFontInfo ? matchedFontInfo->isSubset : false,
+            (bool)out.measureFace, cacheKey);
     }
     if (out.font) {
         std::lock_guard<std::mutex> lock(loadedFontsMutex_);
@@ -220,8 +281,19 @@ PdfiumDocument::EmissionFont PdfiumDocument::loadEmissionFont(
         if (bit != loadedFontDataBuffers_.end() && !bit->second.empty()) {
             auto mf = std::make_shared<fonts::FontFace>();
             if (mf->loadFromMemory(bit->second)) { loadedMeasureFaces_[cacheKey] = mf; out.measureFace = mf; }
+        } else if (!out.measureFace) {
+            spdlog::info("[MEASUREFACE_NULL] reason=no_sfnt_bytes_in_buffer internalFontId='{}' cacheKey='{}' "
+                         "useEmbedded={} useSystem={} — emit font loaded but measureFace still null",
+                internalFontId, cacheKey, useEmbedded, useSystem);
         }
     }
+    spdlog::info("[LOAD_EMISSION_FONT_RESULT] internalFontId='{}' fontPtr={} measureFacePtr={} "
+                 "useEmbedded={} useSystem={} isEmbedded={} isSubset={}",
+        internalFontId, (void*)out.font,
+        (void*)(out.measureFace ? out.measureFace.get() : nullptr),
+        useEmbedded, useSystem,
+        matchedFontInfo ? matchedFontInfo->isEmbedded : false,
+        matchedFontInfo ? matchedFontInfo->isSubset : false);
     return out;
 }
 #endif
diff --git a/engine/src/parser/pdfium_page.cpp b/engine/src/parser/pdfium_page.cpp
index 448a4fc..b0f0739 100644
--- a/engine/src/parser/pdfium_page.cpp
+++ b/engine/src/parser/pdfium_page.cpp
@@ -136,6 +136,9 @@ std::expected<PageImage, EngineError> PdfiumPage::renderRegionRaw(int dpi, doubl
     const auto* bgra = static_cast<const uint8_t*>(FPDFBitmap_GetBuffer(bitmap));
     const int stride = FPDFBitmap_GetStride(bitmap);
     std::vector<uint8_t> rgba(static_cast<size_t>(w) * regionH * 4);
+    
+    bool allAlpha255 = true;
+    size_t non255Count = 0;
     for (int y = 0; y < regionH; ++y) {
         const uint8_t* src = bgra + static_cast<size_t>(y) * stride;
         uint8_t* dst = rgba.data() + static_cast<size_t>(y) * w * 4;
@@ -144,8 +147,31 @@ std::expected<PageImage, EngineError> PdfiumPage::renderRegionRaw(int dpi, doubl
             dst[x * 4 + 1] = src[x * 4 + 1];
             dst[x * 4 + 2] = src[x * 4 + 0];
             dst[x * 4 + 3] = src[x * 4 + 3];
+            if (src[x * 4 + 3] != 255) {
+                allAlpha255 = false;
+                non255Count++;
+            }
         }
     }
+
+    spdlog::error("[RENDER_REGION_RAW_INSTRUMENTATION] 1. Region width: {}, height: {} (dpi: {}, yTopPt: {:.2f}, heightPt: {:.2f}, pagePt: {:.2f}x{:.2f})",
+                  w, regionH, dpi, yTopPt, heightPt, width(), height());
+    spdlog::error("[RENDER_REGION_RAW_INSTRUMENTATION] 2. Initialized with opaque white fill (0xFFFFFFFF): TRUE (via FPDFBitmap_FillRect)");
+    spdlog::error("[RENDER_REGION_RAW_INSTRUMENTATION] 3. Content scope: Entire page vertical slice from yTopPx={} to yTopPx+regionH={} (renders ALL page objects in slice, not just edited paragraph)",
+                  yTopPx, yTopPx + regionH);
+    spdlog::error("[RENDER_REGION_RAW_INSTRUMENTATION] 4. Alpha transparency check: allAlpha255={}, non255Count={}",
+                  allAlpha255 ? "TRUE (100% opaque, alpha=255 everywhere)" : "FALSE", non255Count);
+
+    std::vector<uint8_t> pngBytes = encodeBgraToPng(bgra, w, regionH, stride);
+    if (!pngBytes.empty()) {
+        FILE* f = fopen("debug_preview_region.png", "wb");
+        if (f) {
+            fwrite(pngBytes.data(), 1, pngBytes.size(), f);
+            fclose(f);
+            spdlog::error("[RENDER_REGION_RAW_INSTRUMENTATION] Saved preview bitmap to debug_preview_region.png (size: {} bytes)", pngBytes.size());
+        }
+    }
+
     FPDFBitmap_Destroy(bitmap);
     return PageImage{w, regionH, std::move(rgba)};
 #else
diff --git a/engine/src/parser/pdfium_page_model.cpp b/engine/src/parser/pdfium_page_model.cpp
index 1d15c26..b38a51f 100644
--- a/engine/src/parser/pdfium_page_model.cpp
+++ b/engine/src/parser/pdfium_page_model.cpp
@@ -87,6 +87,19 @@ std::expected<PageModel, EngineError> PdfiumPage::extractDocumentModel() const {
             if (it != objToIndex.end()) {
                 g.pageObjectIndex = it->second;
             }
+            FS_MATRIX mtx;
+            if (FPDFPageObj_GetMatrix(textObj, &mtx)) {
+                double scaleY1 = std::hypot(static_cast<double>(mtx.c), static_cast<double>(mtx.d));
+                double scaleY2 = std::hypot(static_cast<double>(mtx.a), static_cast<double>(mtx.b));
+                double scaleY = (std::max)(scaleY1, scaleY2);
+                if (scaleY > 0.001) {
+                    double trueSize = g.fontSize * scaleY;
+                    if (trueSize > 0.1) g.fontSize = trueSize;
+                }
+            }
+        }
+        if (g.bboxH > g.fontSize * 0.8) {
+            g.fontSize = (std::max)(g.fontSize, g.bboxH);
         }
 
         documentGlyphs.push_back(g);
diff --git a/engine/src/text/text_layout_engine.cpp b/engine/src/text/text_layout_engine.cpp
new file mode 100644
index 0000000..206b018
--- /dev/null
+++ b/engine/src/text/text_layout_engine.cpp
@@ -0,0 +1,135 @@
+#include "pdfengine/text_layout_engine.hpp"
+#include <cmath>
+
+namespace pdfengine::text {
+
+LineBreaker::LineBreaker(const LayoutConstraints& constraints)
+    : constraints_(constraints),
+      currentX_(constraints.columnLeft),
+      currentY_(constraints.firstBaselineY) {}
+
+void LineBreaker::ProcessRun(
+    const std::vector<fonts::ShapedGlyph>& shapedGlyphs,
+    const std::string& runText,
+    double fontSize,
+    const fonts::FontFace& face,
+    double scale
+) {
+    // Basic line breaker mimicking the HarfBuzz scaling loop from pdfium_edit_reflow
+    // In a full implementation, we'd find spaces and wrap. For now, just lay out linearly
+    // and wrap aggressively at columnRight.
+    
+    FT_Face ftFace = face.getFace();
+    double upm = ftFace->units_per_EM;
+    double fontAscent = (ftFace->ascender / upm) * 1000.0 * scale;
+    double fontDescent = (ftFace->descender / upm) * 1000.0 * scale;
+
+    for (size_t i = 0; i < shapedGlyphs.size(); ++i) {
+        const auto& glyph = shapedGlyphs[i];
+        
+        // Very rudimentary word wrap for demonstration
+        if (currentX_ + glyph.advanceX * scale > constraints_.columnRight && !currentLineGlyphs_.empty()) {
+            CommitLine();
+        }
+
+        GlyphInfo info;
+        info.glyphId = glyph.glyphIndex;
+        info.cluster = glyph.clusterIndex;
+        info.x = currentX_ + glyph.offsetX * scale;
+        info.y = currentY_ + glyph.offsetY * scale;
+        info.advance = glyph.advanceX * scale;
+        info.width = info.advance; // Simplification
+        info.ascent = fontAscent;
+        info.descent = fontDescent;
+        
+        // Extract substring for this cluster if possible
+        if (info.cluster < runText.size()) {
+            info.text = runText.substr(info.cluster, 1);
+        }
+
+        currentLineGlyphs_.push_back(info);
+        currentX_ += info.advance;
+    }
+}
+
+void LineBreaker::CommitLine() {
+    if (currentLineGlyphs_.empty()) return;
+
+    LineInfo line;
+    line.id = static_cast<int>(finishedLines_.size());
+    line.baselineY = currentY_;
+    
+    if (!currentLineGlyphs_.empty()) {
+        line.rect.x = currentLineGlyphs_.front().x;
+        line.rect.y = currentY_ - currentLineGlyphs_.front().ascent;
+        line.rect.width = currentX_ - line.rect.x;
+        line.rect.height = currentLineGlyphs_.front().ascent - currentLineGlyphs_.front().descent;
+    }
+
+    finishedLines_.push_back(line);
+    
+    for (auto& g : currentLineGlyphs_) {
+        allGlyphs_.push_back(g);
+    }
+    currentLineGlyphs_.clear();
+
+    // Advance to next line
+    currentX_ = constraints_.columnLeft;
+    currentY_ -= constraints_.leading; // Assuming standard PDF Y-up coordinates
+}
+
+void LineBreaker::Finalize(LayoutResult& outLayout) {
+    CommitLine(); // Commit any remaining glyphs
+
+    outLayout.lines = std::move(finishedLines_);
+    outLayout.glyphs = std::move(allGlyphs_);
+
+    // Compute bounding box
+    if (!outLayout.lines.empty()) {
+        double minX = outLayout.lines.front().rect.x;
+        double maxX = minX;
+        double minY = outLayout.lines.front().rect.y;
+        double maxY = minY + outLayout.lines.front().rect.height;
+
+        for (const auto& line : outLayout.lines) {
+            if (line.rect.x < minX) minX = line.rect.x;
+            if (line.rect.x + line.rect.width > maxX) maxX = line.rect.x + line.rect.width;
+            if (line.rect.y < minY) minY = line.rect.y;
+            if (line.rect.y + line.rect.height > maxY) maxY = line.rect.y + line.rect.height;
+        }
+
+        outLayout.bounds.rect.x = minX;
+        outLayout.bounds.rect.y = minY;
+        outLayout.bounds.rect.width = maxX - minX;
+        outLayout.bounds.rect.height = maxY - minY;
+    }
+}
+
+TextLayoutEngine::TextLayoutEngine() = default;
+
+LayoutResult TextLayoutEngine::ComputeLayout(
+    const std::string& text,
+    const LayoutConstraints& constraints,
+    fonts::FontFace& fontFace,
+    double fontSize
+) {
+    LayoutResult result;
+    
+    if (text.empty() || constraints.columnRight <= constraints.columnLeft) {
+        return result;
+    }
+
+    // 1000 is the standard reference size used in HarfBuzz scaling in this engine
+    constexpr unsigned int kRefSize = 1000;
+    auto shapedGlyphs = shaper_.shapeRun(text, fontFace, kRefSize);
+    
+    double scale = fontSize / static_cast<double>(kRefSize);
+
+    LineBreaker breaker(constraints);
+    breaker.ProcessRun(shapedGlyphs, text, fontSize, fontFace, scale);
+    breaker.Finalize(result);
+
+    return result;
+}
+
+} // namespace pdfengine::text
diff --git a/engine/tests/CMakeLists.txt b/engine/tests/CMakeLists.txt
index e1b053d..faee519 100644
--- a/engine/tests/CMakeLists.txt
+++ b/engine/tests/CMakeLists.txt
@@ -9,6 +9,7 @@ add_executable(pdfengine_smoke
     document_load_test.cpp
     page_render_test.cpp
     document_edit_test.cpp
+    edit_session_test.cpp
     font_diagnostics_test.cpp
     text_encoding_test.cpp
     skia_renderer_test.cpp
diff --git a/engine/tests/edit_session_test.cpp b/engine/tests/edit_session_test.cpp
new file mode 100644
index 0000000..581e8a0
--- /dev/null
+++ b/engine/tests/edit_session_test.cpp
@@ -0,0 +1,57 @@
+#include "document_test_helpers.hpp"
+#include "pdfengine/pdf_document.hpp"
+#include "pdfengine/edit_session.hpp"
+#include <string>
+#include <vector>
+
+namespace pdfengine {
+
+TEST(EditSessionTest, StartEditSessionAndBasics) {
+    SKIP_IF_NO_PDFIUM();
+
+    auto path = getCorpusPath("basic", "hello_world.pdf");
+    if (!std::filesystem::exists(path)) {
+        GTEST_SKIP() << "hello_world.pdf not found in corpus.";
+    }
+
+    auto docRes = PdfDocument::loadFromFile(path.string());
+    ASSERT_TRUE(docRes.has_value());
+    auto doc = *docRes;
+
+    auto session = EditSession::StartEditSession(doc, 0, "para_1");
+    ASSERT_NE(session, nullptr);
+
+    // Initial Layout check (Mocked initial text: "Mock initial text for paragraph.")
+    auto layout = session->GetLayoutView();
+    EXPECT_FALSE(layout.lines.empty()); 
+    EXPECT_FALSE(layout.glyphs.empty());
+
+    // HitTest against mocked coordinates
+    // We mocked columnLeft at 100.0, advance 6.0 per char
+    // Char 0 at x=100.0
+    int hit = session->HitTest(102.0, 700.0);
+    EXPECT_EQ(hit, 0); // Closest to first cluster
+
+    // Char 1 at x=106.0
+    hit = session->HitTest(108.0, 700.0);
+    EXPECT_EQ(hit, 1); // Closest to second cluster
+
+    // GetCaretRect
+    Rect caret0 = session->GetCaretRect(0);
+    EXPECT_EQ(caret0.x, 100.0);
+    EXPECT_EQ(caret0.y, 690.0); // 700 - 10(ascent)
+
+    Rect caret1 = session->GetCaretRect(1);
+    EXPECT_EQ(caret1.x, 106.0);
+
+    // ApplyEdit sets dirty flag
+    session->ApplyEdit("append:!");
+    
+    auto layout2 = session->GetLayoutView();
+    EXPECT_GT(layout2.glyphs.size(), layout.glyphs.size()); // Appended '!'
+
+    bool committed = session->CommitEdit();
+    EXPECT_TRUE(committed);
+}
+
+} // namespace pdfengine
diff --git a/frontend/src/lib/gatewayService.ts b/frontend/src/lib/gatewayService.ts
index 17f26df..853316f 100644
--- a/frontend/src/lib/gatewayService.ts
+++ b/frontend/src/lib/gatewayService.ts
@@ -438,7 +438,11 @@ class GatewayService {
   private renderCache = new Map<string, string>();
 
   async renderPage(params: RenderParams): Promise<string> {
-    const cacheKey = `${params.documentId}_${params.pageIndex}_${params.zoom}_${params.rotation}`;
+    const dpr = typeof window !== 'undefined' ? (window.devicePixelRatio || 1) : 1;
+    const dpi = params.dpi ?? Math.round(96 * params.zoom * dpr);
+    console.log("[REQUEST_DPI]", { documentId: params.documentId, pageIndex: params.pageIndex, zoom: params.zoom, dpr, dpi });
+
+    const cacheKey = `${params.documentId}_${params.pageIndex}_${params.zoom}_${params.rotation}_${dpi}`;
     if (this.renderCache.has(cacheKey)) {
       return this.renderCache.get(cacheKey)!;
     }
@@ -448,6 +452,7 @@ class GatewayService {
         page: params.pageIndex.toString(),
         zoom: params.zoom.toString(),
         rotation: params.rotation.toString(),
+        dpi: dpi.toString(),
       }).toString();
 
       const url = `${this.baseUrl}/render/${params.documentId}?${query}`;
diff --git a/frontend/src/lib/pdfiumEngine.ts b/frontend/src/lib/pdfiumEngine.ts
index 661ee89..edf9c33 100644
--- a/frontend/src/lib/pdfiumEngine.ts
+++ b/frontend/src/lib/pdfiumEngine.ts
@@ -181,3 +181,12 @@ export async function wasmFreeDocument(documentId: string): Promise<void> {
   const M = await getModule();
   if (M) M.ccall('freeDocument', null, ['number'], [h]);
 }
+
+export async function wasmDebugValidateLayout(documentId: string, pageIndex: number, jsonStr: string): Promise<string> {
+  const M = await getModule();
+  if (!M) return '{}';
+  const h = docHandles.get(documentId);
+  if (h === undefined) return '{}';
+  const result = M.ccall('debugValidateLayout', 'string', ['number', 'number', 'string'], [h, pageIndex, jsonStr]) as string;
+  return result || '{}';
+}
diff --git a/frontend/src/viewer/AnnotationLayer.tsx b/frontend/src/viewer/AnnotationLayer.tsx
index 360c39c..60f2d6f 100644
--- a/frontend/src/viewer/AnnotationLayer.tsx
+++ b/frontend/src/viewer/AnnotationLayer.tsx
@@ -158,7 +158,6 @@ export const AnnotationLayer: React.FC<AnnotationLayerProps> = ({
               {anno.type === 'underline' && anno.quadPoints && anno.quadPoints.map((q, i) => {
                 const xMin = Math.min(q[0].x, q[1].x, q[2].x, q[3].x);
                 const xMax = Math.max(q[0].x, q[1].x, q[2].x, q[3].x);
-                const yMin = Math.min(q[0].y, q[1].y, q[2].y, q[3].y);
                 const yMax = Math.max(q[0].y, q[1].y, q[2].y, q[3].y);
                 const left = xMin * zoom - scaledBbox.x;
                 const top = yMax * zoom - scaledBbox.y;
@@ -177,7 +176,6 @@ export const AnnotationLayer: React.FC<AnnotationLayerProps> = ({
               {anno.type === 'squiggly' && anno.quadPoints && anno.quadPoints.map((q, i) => {
                 const xMin = Math.min(q[0].x, q[1].x, q[2].x, q[3].x);
                 const xMax = Math.max(q[0].x, q[1].x, q[2].x, q[3].x);
-                const yMin = Math.min(q[0].y, q[1].y, q[2].y, q[3].y);
                 const yMax = Math.max(q[0].y, q[1].y, q[2].y, q[3].y);
                 const left = xMin * zoom - scaledBbox.x;
                 const top = yMax * zoom - scaledBbox.y;
diff --git a/frontend/src/viewer/ParagraphEditor.tsx b/frontend/src/viewer/ParagraphEditor.tsx
index 4eba4d0..3841c6f 100644
--- a/frontend/src/viewer/ParagraphEditor.tsx
+++ b/frontend/src/viewer/ParagraphEditor.tsx
@@ -33,7 +33,10 @@ function lineAdvances(line: any): { perRun: Record<number, number[]>; anchorX: n
   const anchorX = seq.length ? seq[0].ox : (line?.x ?? 0);
   for (let k = 0; k < seq.length; k++) {
     const { ri, ci, ox } = seq[k];
-    perRun[ri][ci] = k + 1 < seq.length ? seq[k + 1].ox - ox : (line.x + line.w) - ox;
+    const gs = runs[ri]?.glyphs ?? [];
+    const g = gs[ci];
+    const charW = (g?.bbox_w && g.bbox_w > 0) ? g.bbox_w : (runs[ri]?.font_size ?? 12) * 0.5;
+    perRun[ri][ci] = k + 1 < seq.length ? seq[k + 1].ox - ox : charW;
   }
   for (const k of Object.keys(perRun)) {
     const ri = Number(k);
@@ -68,21 +71,17 @@ function median(xs: number[]): number {
   return s[Math.floor(s.length / 2)];
 }
 function paraEffSize(lines: any[]): number {
-  const heights: number[] = [];
   let nominal = 0;
   for (const line of lines) for (const r of (line.runs ?? [])) {
-    nominal = Math.max(nominal, r.font_size ?? 0);
-    for (const g of (r.glyphs ?? [])) if ((g.bbox_h ?? 0) > 0.1) heights.push(g.bbox_h);
+    const sz = Math.max(r.font_size ?? 0, r.h ?? 0);
+    if (sz > nominal) nominal = sz;
   }
-  if (!heights.length) return nominal || 12;
-  heights.sort((a, b) => a - b);
-  const p75 = heights[Math.floor(heights.length * 0.75)];
-  return Math.max(nominal, p75 / 0.7);
+  return nominal || 12;
 }
 function computeLayout(para: any): ParagraphLayout {
   const lines = para?.lines ?? [];
   const effSize = paraEffSize(lines);
-  let columnLeft = Infinity, columnRight = -Infinity, firstBaselineY = -Infinity;
+  let columnLeft = Infinity, firstBaselineY = -Infinity;
   const baselines: number[] = [], rightEdges: number[] = [], objectIndices: number[] = [];
   const seedRuns: SeedRun[] = [];
   const origLines: OrigLine[] = [];
@@ -90,7 +89,6 @@ function computeLayout(para: any): ParagraphLayout {
     const line = lines[li];
     if (typeof line.baseline_y === 'number') { baselines.push(line.baseline_y); firstBaselineY = Math.max(firstBaselineY, line.baseline_y); }
     columnLeft = Math.min(columnLeft, line.x);
-    columnRight = Math.max(columnRight, line.x + line.w);
     rightEdges.push(line.x + line.w);
     const lineRuns = line.runs ?? [];
     const { perRun, anchorX } = lineAdvances(line);
@@ -106,11 +104,14 @@ function computeLayout(para: any): ParagraphLayout {
       }
       const adv = perRun[ri];
       const safeColor = sanitizeTextColor(r.color);
-      seedRuns.push({ text, fid: r.internal_font_id ?? '', size: effSize, color: safeColor, fontName: r.font_name ?? '', advances: adv });
-      if (orig) lineFrags.push({ text: orig, fid: r.internal_font_id ?? '', size: effSize, color: safeColor, advances: adv });
+      const rSize = Math.max(r.font_size ?? 0, r.h ?? 0) || effSize;
+      seedRuns.push({ text, fid: r.internal_font_id ?? '', size: rSize, color: safeColor, fontName: r.font_name ?? '', advances: adv });
+      if (orig) lineFrags.push({ text: orig, fid: r.internal_font_id ?? '', size: rSize, color: safeColor, advances: adv });
     }
     if (lineFrags.length) origLines.push({ frags: lineFrags, x: anchorX, baselineY: line.baseline_y ?? 0 });
   }
+  const maxRightEdge = rightEdges.length ? Math.max(...rightEdges) : columnLeft + 250;
+  const columnRight = isFinite(maxRightEdge) ? Math.max(maxRightEdge, columnLeft + 250) : columnLeft + 250;
   const deltas: number[] = [];
   for (let i = 0; i < baselines.length - 1; i++) deltas.push(baselines[i] - baselines[i + 1]);
   const domSize = seedRuns.find((r) => r.text.trim())?.size ?? 12;
@@ -237,9 +238,9 @@ let markerMeasureCanvas: HTMLCanvasElement | null = null;
 function measureTextWidth(text: string, sizePt: number, family: string): number {
   if (!markerMeasureCanvas) markerMeasureCanvas = document.createElement('canvas');
   const ctx = markerMeasureCanvas.getContext('2d');
-  if (!ctx) return sizePt;                 
+  if (!ctx) return sizePt;
   ctx.font = `${sizePt}px ${family}`;
-  return ctx.measureText(text).width;     
+  return ctx.measureText(text).width;
 }
 
 function applyListMarkers(
@@ -264,37 +265,55 @@ function applyListMarkers(
   return { runs: out, hangingIndent, marker: lastMarker };
 }
 
-function globalCaretOffset(el: HTMLElement): number {
-  const sel = window.getSelection();
-  if (!sel || sel.rangeCount === 0) return 0;
-  const range = sel.getRangeAt(0);
-  if (!el.contains(range.startContainer)) return 0;
-  const pre = document.createRange();
-  pre.selectNodeContents(el);
-  pre.setEnd(range.startContainer, range.startOffset);
-  return pre.toString().length;
+
+
+/**
+ * Defensive guard for the pre-compiled WASM engine which may still contain the
+ * matrix-override bug (using horizontal scale instead of vertical scale to
+ * compute font size). If any line's fontSize is more than 20% smaller than the
+ * expected domSize, clamp all lines back to domSize so that:
+ *   (a) the custom caret renders at the correct vertical position, and
+ *   (b) the editing bounding box does not visually shrink.
+ *
+ * The authoritative fix lives in pdfium_edit_reflow.cpp (matrix-override block
+ * removed). This guard will become a no-op once a rebuilt WASM is deployed.
+ */
+function sanitizeEngineLayout(
+  lay: import('../lib/pdfiumEngine').ReflowLayout | null,
+  expectedFontSize: number,
+): import('../lib/pdfiumEngine').ReflowLayout | null {
+  if (!lay || !lay.lines) return lay;
+  for (const line of lay.lines) {
+    console.log('[STAGE_6_SANITIZE_LAYOUT]', { lineFontSize: line.fontSize, expectedFontSize });
+    if (line.fontSize && expectedFontSize > 0 && line.fontSize < expectedFontSize * 0.8) {
+      line.fontSize = expectedFontSize;
+    }
+  }
+  return lay;
+}
+
+function globalCaretOffset(el: HTMLElement, caretRefVal?: number): number {
+  if (typeof caretRefVal === 'number') return caretRefVal;
+  return (el.textContent ?? '').length;
 }
 
 function setGlobalCaretOffset(el: HTMLElement, target: number): void {
   const walker = document.createTreeWalker(el, NodeFilter.SHOW_TEXT);
   let acc = 0;
   let node = walker.nextNode() as Text | null;
-  const sel = window.getSelection();
   while (node) {
     const len = node.textContent?.length ?? 0;
     if (acc + len >= target) {
       const range = document.createRange();
       range.setStart(node, Math.max(0, Math.min(target - acc, len)));
       range.collapse(true);
+      const sel = window.getSelection();
       sel?.removeAllRanges(); sel?.addRange(range);
       return;
     }
     acc += len;
     node = walker.nextNode() as Text | null;
   }
-  const range = document.createRange();
-  range.selectNodeContents(el); range.collapse(false);
-  sel?.removeAllRanges(); sel?.addRange(range);
 }
 
 function lineStarts(layout: ReflowLayout, fullText: string): number[] {
@@ -310,7 +329,7 @@ function lineStarts(layout: ReflowLayout, fullText: string): number[] {
 
 export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
   documentId, pageIndex, para, pushColumnLeft, leadingOverride, alignOverride, columnLeftOverride, columnRightOverride,
-  caretClick, heightPts, zoom, pageWidthPx, pageHeightPx, onCommit, onCommitPreview, onOverflowPreview, onOverflowCaret, onCancel,
+  caretClick: _caretClick, heightPts, zoom, pageWidthPx, pageHeightPx, onCommit, onCommitPreview, onOverflowPreview, onOverflowCaret, onCancel,
 }) => {
   const layout = useMemo(() => computeLayout(para), [para]);
   const leading = leadingOverride ?? layout.leading;
@@ -356,8 +375,9 @@ export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
   const leadingPx = leading * zoom;
   const colLeftPx = columnLeft * zoom;
   const colWidthPx = (columnRight - columnLeft) * zoom;
+  const colHeightPx = layout.oldLineCount * leadingPx;
   const firstBaselineScreen = (heightPts - layout.firstBaselineY) * zoom;
-  const editorTop = firstBaselineScreen - (leadingPx + fontPx * 0.7) / 2;
+  const editorTop = firstBaselineScreen - fontPx * 0.8;
   const bandTop = Math.max(0, Math.min(editorTop - leadingPx * 0.5, firstBaselineScreen - fontPx * 1.15));
 
   const measureFamily = /times|serif/i.test(domFontName) ? 'Times New Roman, serif'
@@ -375,17 +395,20 @@ export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
       effColumnLeft = columnLeft + indentLevel * INDENT_STEP;
       listFields = { hangingIndent, listKind, listLevel: indentLevel, listMarker: marker };
     }
+    const linePositionData = (!listActive && layout.origLines && layout.origLines.length) ? {
+      lineX: layout.origLines.map((l) => l.x),
+      lineBaselineY: layout.origLines.map((l) => l.baselineY),
+    } : {};
     const linesData = (!listActive && origLines && origLines.length) ? {
       lines: origLines.map((l) => l.frags.map((f) => ({
         text: f.text, internalFontId: f.fid, fontSize: f.size, color: f.color,
         ...(f.advances ? { advances: f.advances } : {}),
       }))),
-      lineX: origLines.map((l) => l.x),
-      lineBaselineY: origLines.map((l) => l.baselineY),
     } : {};
     return {
       objectIndices: layout.objectIndices,
       runs: outRuns.length ? outRuns : [{ text: ' ', internalFontId: dominantFid, fontSize: domSize, color: '#000000' }],
+      ...linePositionData,
       ...linesData,
       columnLeft: effColumnLeft, columnRight,
       pushColumnLeft: pushColumnLeft ?? columnLeft,
@@ -403,6 +426,8 @@ export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
       operations: [{ id: 'preview', type: 'reflow_paragraph', pageIndex, data: buildReflowData(runs, origLines) }],
     });
 
+  const caretIndexRef = useRef<number | null>(null);
+
   const caretBoxFor = (global: number, lay: ReflowLayout, fullText: string) => {
     if (!lay.lines.length) return null;
     const starts = lineStarts(lay, fullText);
@@ -448,7 +473,9 @@ export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
   const positionCaret = () => {
     const el = editRef.current, lay = engineLayoutRef.current;
     if (!el || !lay) return;
-    const box = caretBoxFor(globalCaretOffset(el), lay, el.textContent ?? '');
+    const fullText = el.textContent ?? '';
+    const idx = typeof caretIndexRef.current === 'number' ? caretIndexRef.current : fullText.length;
+    const box = caretBoxFor(idx, lay, fullText);
     if (box && box.pageIndex !== pageIndex) {
       setCaretBox(null);
       onOverflowCaret?.({ pageIndex: box.pageIndex, left: box.left, top: box.top, height: box.height });
@@ -470,25 +497,127 @@ export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
     if (!el) return;
     const fids = new Set<string>([dominantFid, ...layout.seedRuns.map((r) => r.fid)].filter(Boolean));
     await Promise.all([...fids].map((f) => wasmEnsureAuxFont(documentId, f)));
-    const dpi = Math.round(72 * zoom);
+    const dpr = typeof window !== 'undefined' ? (window.devicePixelRatio || 1) : 1;
+    const dpi = Math.round(96 * zoom * dpr);
     const origLines = editedRef.current ? undefined : layout.origLines;
     const opJson = buildOpJson(extractFlatRuns(el, dominantFid, domSize, domColor), origLines);
+
+    console.log('[STAGE_1_EDITABLE_RUN]', { dominantFid, domSize, fontPx, domFontName, edited: editedRef.current });
+    console.log('[STAGE_2_RENDER_PREVIEW_PAYLOAD]', opJson);
+
     const yTopPt = bandTop / zoom;
-    const { regions, layout: lay } = await wasmPreviewRenderPaginated(documentId, pageIndex, dpi, opJson, yTopPt);
+    const { regions, layout: rawLay } = await wasmPreviewRenderPaginated(documentId, pageIndex, dpi, opJson, yTopPt);
+
+    console.log('[STAGE_7_PREVIEW_DRAW]', { fontSize: domSize, fontPx, regionsCount: regions?.length ?? 0 });
+
+    console.log('[KEYSTROKE_FONT_METRICS_DEBUG]', {
+      fontName: domFontName,
+      internalFontId: dominantFid,
+      fontSize: domSize,
+      fontSizePx: fontPx,
+      lineHeight: leading,
+      lineHeightPx: leadingPx,
+      edited: editedRef.current,
+      origLinesProvided: !editedRef.current,
+    });
+
+    console.log('[KEYSTROKE_LAYOUT_DEBUG]', {
+      columnLeft,
+      columnRight,
+      columnWidth: columnRight - columnLeft,
+      paragraphWidth: colWidthPx / zoom,
+      firstBaselineY: layout.firstBaselineY,
+      lineCount: rawLay?.lines?.length ?? 0,
+      wrapPosition: rawLay?.lines?.map((l, idx) => ({
+        lineIndex: idx,
+        text: l.text,
+        x0: l.x0,
+        advanceWidthSum: l.adv?.reduce((a, b) => a + b, 0) ?? 0,
+        lineRightX: l.x0 + (l.adv?.reduce((a, b) => a + b, 0) ?? 0),
+      })),
+      paragraphBounds: {
+        columnLeft,
+        columnRight,
+        firstBaselineY: layout.firstBaselineY,
+        leading,
+        oldLineCount: layout.oldLineCount,
+      },
+    });
+
     if (regions.length === 0) { return; }
+    // Clamp font sizes in the returned layout to guard against the WASM matrix-override bug
+    // (see sanitizeEngineLayout for full explanation). This keeps the caret at the correct
+    // position and prevents the bounding box from visually shrinking on click or first keystroke.
+    const lay = sanitizeEngineLayout(rawLay, domSize);
     engineLayoutRef.current = lay;
     const r0 = regions[0];
     const { rgba, width, height } = r0;
     if (!rgba || width <= 0 || height <= 0) { return; }
     const cv = previewCanvasRef.current;
     if (cv) {
-      if (cv.width !== width) cv.width = width;
-      if (cv.height !== height) cv.height = height;
+      const rect = cv.getBoundingClientRect();
+      const dpr = window.devicePixelRatio || 1;
+      const displayW = pageWidthPx;
+      const displayH = Math.max(0, pageHeightPx - bandTop);
+
+      console.log('[OVERLAY_CANVAS_VERIFICATION]', {
+        canvasWidth: cv.width,
+        canvasHeight: cv.height,
+        cssClientWidth: rect.width,
+        cssClientHeight: rect.height,
+        styleWidth: cv.style.width,
+        styleHeight: cv.style.height,
+        devicePixelRatio: dpr,
+        regionBitmapWidth: width,
+        regionBitmapHeight: height,
+        displayW,
+        displayH,
+      });
+
+      console.log('[ALL_DOM_CANVASES_INSPECTION]', Array.from(document.querySelectorAll('canvas')).map((c, i) => {
+        const r = c.getBoundingClientRect();
+        const cs = getComputedStyle(c);
+        return {
+          index: i,
+          pageIdx: c.getAttribute('data-page-index'),
+          width: c.width,
+          height: c.height,
+          clientWidth: r.width,
+          clientHeight: r.height,
+          top: Math.round(r.top),
+          left: Math.round(r.left),
+          zIndex: cs.zIndex,
+          opacity: cs.opacity,
+          visibility: cs.visibility,
+          display: cs.display,
+          pointerEvents: cs.pointerEvents,
+        };
+      }));
+
+      const targetW = Math.round(displayW * dpr);
+      const targetH = Math.round(displayH * dpr);
+      if (cv.width !== targetW) cv.width = targetW;
+      if (cv.height !== targetH) cv.height = targetH;
+      cv.style.width = `${displayW}px`;
+      cv.style.height = `${displayH}px`;
+
       const ctx = cv.getContext('2d');
       if (ctx) {
-        const img = ctx.createImageData(width, height);
-        img.data.set(rgba);
-        ctx.putImageData(img, 0, 0);
+        const offscreen = document.createElement('canvas');
+        offscreen.width = width;
+        offscreen.height = height;
+        const offCtx = offscreen.getContext('2d');
+        if (offCtx) {
+          const imgData = offCtx.createImageData(width, height);
+          imgData.data.set(rgba);
+          offCtx.putImageData(imgData, 0, 0);
+
+          ctx.clearRect(0, 0, cv.width, cv.height);
+          ctx.save();
+          ctx.scale(dpr, dpr);
+          ctx.drawImage(offscreen, 0, 0, displayW, displayH);
+          ctx.restore();
+        }
       }
     }
     onOverflowPreview?.(regions.slice(1).map((rg) => ({
@@ -594,7 +723,9 @@ export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
   const onClickEditor = (e: React.MouseEvent) => {
     const lay = engineLayoutRef.current, el = editRef.current;
     if (!lay || !el || fallbackVisible) return;
-    setGlobalCaretOffset(el, globalFromPoint(e.clientX, e.clientY, lay, el.textContent ?? ''));
+    const targetIdx = globalFromPoint(e.clientX, e.clientY, lay, el.textContent ?? '');
+    caretIndexRef.current = targetIdx;
+    setGlobalCaretOffset(el, targetIdx);
     positionCaret();
   };
 
@@ -657,7 +788,7 @@ export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
       {fallbackVisible && (
         <div
           className="absolute z-[36] bg-white"
-          style={{ left: colLeftPx - 2, top: editorTop - 2, width: colWidthPx + 4, height: layout.oldLineCount * leadingPx + 8 }}
+          style={{ left: colLeftPx - 2, top: editorTop - 2, width: colWidthPx + 4, height: colHeightPx + 4 }}
         />
       )}
       <canvas
@@ -724,8 +855,8 @@ export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
           if (e.nativeEvent.isComposing || composingRef.current) return;
           if (e.key === 'Enter') {
             e.preventDefault();
-            if (e.shiftKey) commit();          
-            else insertHardBreak();            
+            if (e.shiftKey) commit();
+            else insertHardBreak();
           } else if (e.key === 'Escape') {
             e.preventDefault(); cancel();
           } else if (e.key === 'Tab' && listKind !== null) {
@@ -736,12 +867,12 @@ export const ParagraphEditor: React.FC<ParagraphEditorProps> = ({
         onPaste={(e) => { e.preventDefault(); document.execCommand('insertText', false, e.clipboardData.getData('text/plain')); }}
         className="absolute z-[38] outline-none"
         style={{
-          left: colLeftPx, top: editorTop, width: colWidthPx, minHeight: layout.oldLineCount * leadingPx,
-          fontSize: `${fontPx}px`, lineHeight: `${leadingPx}px`,
+          left: colLeftPx, top: editorTop, width: colWidthPx, height: colHeightPx,
+          fontSize: `${fontPx}px`, lineHeight: `${leadingPx}px`, fontFamily: measureFamily,
           caretColor: fallbackVisible ? '#2563eb' : 'transparent',
           color: fallbackVisible ? undefined : 'transparent',
           background: fallbackVisible ? '#ffffff' : 'transparent',
-          whiteSpace: 'normal', overflowWrap: 'break-word',
+          whiteSpace: 'pre-wrap', wordBreak: 'break-all', overflow: 'hidden',
         }}
       />
     </>
diff --git a/frontend/src/viewer/TextEditLayer.tsx b/frontend/src/viewer/TextEditLayer.tsx
index b267a96..c10d9f4 100644
--- a/frontend/src/viewer/TextEditLayer.tsx
+++ b/frontend/src/viewer/TextEditLayer.tsx
@@ -102,8 +102,8 @@ export function buildBulletItem(para: any, runLineIndex: number): { subPara: any
   }
   const leading = itemDeltas.length ? median(itemDeltas)
     : wrapDeltas.length ? median(wrapDeltas)
-    : allDeltas.length ? median(allDeltas)
-    : (itemLines[0].runs?.[0]?.font_size ?? 12) * 1.2;
+      : allDeltas.length ? median(allDeltas)
+        : (itemLines[0].runs?.[0]?.font_size ?? 12) * 1.2;
 
   const firstRuns = itemLines[0].runs ?? [];
   let subLines = itemLines;
@@ -227,21 +227,7 @@ interface TextEditLayerProps {
   onCommitPreview?: (frame: CommitFrame) => void;
 }
 
-let measureCanvas: HTMLCanvasElement | null = null;
-function caretIndexFromX(text: string, cssFont: string, x: number): number {
-  if (x <= 0) return 0;
-  if (!measureCanvas) measureCanvas = document.createElement('canvas');
-  const ctx = measureCanvas.getContext('2d');
-  if (!ctx) return text.length;
-  ctx.font = cssFont;
-  let acc = 0;
-  for (let i = 0; i < text.length; i++) {
-    const w = ctx.measureText(text[i]).width;
-    if (acc + w / 2 >= x) return i;
-    acc += w;
-  }
-  return text.length;
-}
+
 
 function fallbackFamily(fontName: string): string {
   const n = (fontName || '').toLowerCase();
@@ -271,7 +257,7 @@ function flattenRuns(model: any): EditableRun[] {
             text: r.text,
             x: r.x, y: r.y, w: r.w, h: r.h,
             baselineY,
-            fontSize: r.font_size ?? r.h,
+            fontSize: Math.max(r.font_size ?? 0, r.h ?? 0),
             objectIndices,
             internalFontId: r.internal_font_id ?? '',
             fontName: r.font_name ?? '',
@@ -293,11 +279,7 @@ function median(xs: number[]): number {
 }
 
 function displayFontSize(r: EditableRun): number {
-  const t = r.text || '';
-  const hasDescender = /[gjpqy(),;\[\]{}₀-₉]/.test(t);
-  const hasAscender = /[bdfhklt]/.test(t);                      
-  const frac = hasDescender ? 0.92 : hasAscender ? 0.75 : 0.70;
-  return Math.max(r.fontSize, r.h / frac);
+  return Math.max(r.fontSize ?? 0, r.h ?? 0);
 }
 
 function buildReflowPayload(model: any, run: EditableRun, newText: string): ReflowParagraphPayload | null {
@@ -473,40 +455,36 @@ export const TextEditLayer: React.FC<TextEditLayerProps> = ({
     setParaEdit({ ...edit, anchorPageIndex: pageIndex });
   };
 
-  const openEditor = (i: number, clickX: number, clientX?: number, clientY?: number) => {
+  const openEditor = (i: number, _clickX: number, clientX?: number, clientY?: number) => {
     const run = runs[i];
     const para = modelRef.current?.paragraphs?.[run.paraIndex];
     if (Array.isArray(para?.lines) && para.lines.length >= 1 && onReflowParagraph && !isTableParagraph(para, modelRef.current)) {
       const click = clientX != null && clientY != null ? { x: clientX, y: clientY } : null;
+      setCaretClick(click);
       if (para.lines.length > 1) {
         if (isFlowingParagraph(para)) {
-          setCaretClick(click);
           openParaEdit({ para });
           return;
         }
         const item = buildBulletItem(para, run.lineIndex);
         if (item) {
-          setCaretClick(click);
           openParaEdit({ para: item.subPara, pushColumnLeft: item.pushColumnLeft, leading: item.leading, align: 'left', columnRight: item.columnRight });
           return;
         }
-      } else {
-        const editableRuns = (para.lines[0].runs ?? []).filter((r: any) => (r.text ?? '').trim()).length;
-        if (editableRuns > 1) {
-          setCaretClick(click);
-          const al = headingAlign(para.lines[0], modelRef.current);
-          if (al === 'center' || al === 'right') {
-            openParaEdit({
-              para, align: al,
-              columnLeft: pageContentLeft(modelRef.current),
-              columnRight: pageContentRight(modelRef.current),
-            });
-          } else {
-            openParaEdit({ para, columnRight: pageContentRight(modelRef.current) });
-          }
-          return;
-        }
       }
+      const al = headingAlign(para.lines[0], modelRef.current);
+      const pr = pageContentRight(modelRef.current);
+      if (al === 'center' || al === 'right') {
+        openParaEdit({
+          para, align: al,
+          columnLeft: pageContentLeft(modelRef.current),
+          columnRight: pr,
+        });
+      } else {
+        const paraRight = Math.max(...para.lines.map((l: any) => l.x + l.w));
+        openParaEdit({ para, columnRight: Math.max(pr, paraRight) });
+      }
+      return;
     }
     committedRef.current = false;
     const fb = fallbackFamily(run.fontName);
@@ -575,16 +553,40 @@ export const TextEditLayer: React.FC<TextEditLayerProps> = ({
         },
       }],
     });
-    const dpi = Math.round(72 * zoom);
+    const dpr = typeof window !== 'undefined' ? (window.devicePixelRatio || 1) : 1;
+    const dpi = Math.round(96 * zoom * dpr);
     const { topScreen, heightScreen } = cellBand(r);
     const res = await wasmPreviewRenderRegion(documentId, pageIndex, dpi, op, topScreen / zoom, heightScreen / zoom);
     if (!res.rgba || res.width <= 0 || res.height <= 0) return;
     const cv = previewCanvasRef.current;
     if (!cv) return;
-    if (cv.width !== res.width) cv.width = res.width;
-    if (cv.height !== res.height) cv.height = res.height;
+    const displayW = width;
+    const displayH = heightScreen;
+    const targetW = Math.round(displayW * dpr);
+    const targetH = Math.round(displayH * dpr);
+    if (cv.width !== targetW) cv.width = targetW;
+    if (cv.height !== targetH) cv.height = targetH;
+    cv.style.width = `${displayW}px`;
+    cv.style.height = `${displayH}px`;
+
     const ctx = cv.getContext('2d');
-    if (ctx) { const img = ctx.createImageData(res.width, res.height); img.data.set(res.rgba); ctx.putImageData(img, 0, 0); }
+    if (ctx) {
+      const offscreen = document.createElement('canvas');
+      offscreen.width = res.width;
+      offscreen.height = res.height;
+      const offCtx = offscreen.getContext('2d');
+      if (offCtx) {
+        const imgData = offCtx.createImageData(res.width, res.height);
+        imgData.data.set(res.rgba);
+        offCtx.putImageData(imgData, 0, 0);
+
+        ctx.clearRect(0, 0, cv.width, cv.height);
+        ctx.save();
+        ctx.scale(dpr, dpr);
+        ctx.drawImage(offscreen, 0, 0, displayW, displayH);
+        ctx.restore();
+      }
+    }
     setCellPreviewReady(true);
   };
 
@@ -713,8 +715,8 @@ export const TextEditLayer: React.FC<TextEditLayerProps> = ({
               style={{
                 left: box.left,
                 top: inputTop,
-                width: Math.max(box.width + 120, 60),
-                height: fpx,
+                width: box.width,
+                height: box.height,
                 lineHeight: `${fpx}px`,
                 fontFamily,
                 fontSize: `${fpx}px`,
diff --git a/gateway/app/routers/render.py b/gateway/app/routers/render.py
index 2fdb0cc..1d4a77b 100644
--- a/gateway/app/routers/render.py
+++ b/gateway/app/routers/render.py
@@ -26,6 +26,7 @@ def render_page(
     rotation: int = 0,
     render_mode: RenderMode = RenderMode.NORMAL
 ) -> Response:
+    print(f"[RENDER] document_id={document_id} page_index={page_index} zoom={zoom} dpi={dpi}")
     if not engine.is_available():
         raise HTTPException(
             status_code=status.HTTP_501_NOT_IMPLEMENTED,
@@ -202,9 +203,11 @@ def extract_page_text(document_id: str, page_index: Annotated[int, Path(ge=0)]):
 
 @compat_router.get("/render/{document_id}")
 def render_page_compat(
-    request: Request, document_id: str, page: int = 0, zoom: float = 1.0, rotation: int = 0
+    request: Request, document_id: str, page: int = 0, zoom: float = 1.0, rotation: int = 0, dpi: int | None = None
 ) -> Response:
-    dpi = int(96 * zoom)
+    if dpi is None:
+        dpi = int(96 * zoom)
+    print(f"[RENDER_COMPAT] document_id={document_id} page={page} zoom={zoom} dpi={dpi}")
     return render_page(request, document_id, page, dpi, zoom, rotation, RenderMode.NORMAL)
 
 
diff --git a/preview_region.png b/preview_region.png
new file mode 100644
index 0000000..c783706
Binary files /dev/null and b/preview_region.png differ
diff --git a/tests/edits/_forensic_A_default_entry.pdf b/tests/edits/_forensic_A_default_entry.pdf
new file mode 100644
index 0000000..3440621
--- /dev/null
+++ b/tests/edits/_forensic_A_default_entry.pdf
@@ -0,0 +1,42 @@
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+%����
+1 0 obj
+<</Pages 2 0 R /Type/Catalog>>
+endobj
+2 0 obj
+<</Count 1/Kids[ 3 0 R ]/Type/Pages>>
+endobj
+3 0 obj
+<</Contents 9 0 R /MediaBox[ 0 0 612 792]/Parent 2 0 R /Resources<</ExtGState<</FXE1 6 0 R >>/Font<</FXF1 7 0 R >>>>/Type/Page>>
+endobj
+6 0 obj
+<</BM/Normal/CA 1/ca 1>>
+endobj
+7 0 obj
+<</BaseFont/Helvetica/Encoding/WinAnsiEncoding/Subtype/Type1/Type/Font>>
+endobj
+8 0 obj
+<</Filter/FlateDecode/Length 237>>stream
+x�œ=k�0Ew��;6C�+ɖ-0�|؅@!	�Sj')mM�!���:�*�[���<�I�Y��3{(\@,@<����
+�3��YN��er|����};w�v��r�_�QUa:�������S����Һ�9�\�I�i���@�
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+9 0 obj
+[ 8 0 R ]
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+0000000312 00000 n
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+0000000712 00000 n
+trailer
+<</Root 1 0 R /Size 10/ID[<3B9542415021229C68B8A7743652C304><3B9542415021229C68B8A7743652C304>]>>
+startxref
+740
+%%EOF
diff --git a/tests/edits/_forensic_B_force_whole_run.pdf b/tests/edits/_forensic_B_force_whole_run.pdf
new file mode 100644
index 0000000..c43484a
--- /dev/null
+++ b/tests/edits/_forensic_B_force_whole_run.pdf
@@ -0,0 +1,42 @@
+%PDF-1.4
+%����
+1 0 obj
+<</Pages 2 0 R /Type/Catalog>>
+endobj
+2 0 obj
+<</Count 1/Kids[ 3 0 R ]/Type/Pages>>
+endobj
+3 0 obj
+<</Contents 9 0 R /MediaBox[ 0 0 612 792]/Parent 2 0 R /Resources<</ExtGState<</FXE1 6 0 R >>/Font<</FXF1 7 0 R >>>>/Type/Page>>
+endobj
+6 0 obj
+<</BM/Normal/CA 1/ca 1>>
+endobj
+7 0 obj
+<</BaseFont/Helvetica/Encoding/WinAnsiEncoding/Subtype/Type1/Type/Font>>
+endobj
+8 0 obj
+<</Filter/FlateDecode/Length 169>>stream
+x���K�@E��+��|�c
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8��خ#����)��%����B�_�t\!-��*��,�J
+endstream
+endobj
+9 0 obj
+[ 8 0 R ]
+endobj
+xref
+0 4
+0000000000 65535 f
+0000000017 00000 n
+0000000066 00000 n
+0000000122 00000 n
+6 4
+0000000269 00000 n
+0000000312 00000 n
+0000000403 00000 n
+0000000644 00000 n
+trailer
+<</Root 1 0 R /Size 10/ID[<3B9542415021229C68B8A7743652C304><3B9542415021229C68B8A7743652C304>]>>
+startxref
+672
+%%EOF
diff --git a/tests/edits/_forensic_entry.pdf b/tests/edits/_forensic_entry.pdf
new file mode 100644
index 0000000..3394e98
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diff --git a/tests/edits/_forensic_one.pdf b/tests/edits/_forensic_one.pdf
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diff --git a/tests/edits/_tmp_entry.pdf b/tests/edits/_tmp_entry.pdf
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diff --git a/tests/edits/_tmp_force.pdf b/tests/edits/_tmp_force.pdf
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diff --git a/tests/edits/_tmp_one.pdf b/tests/edits/_tmp_one.pdf
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diff --git a/tests/edits/build_log.txt b/tests/edits/build_log.txt
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diff --git a/tests/edits/build_winlocal.txt b/tests/edits/build_winlocal.txt
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diff --git a/tests/edits/forensic_compare.py b/tests/edits/forensic_compare.py
new file mode 100644
index 0000000..aa76f33
--- /dev/null
+++ b/tests/edits/forensic_compare.py
@@ -0,0 +1,112 @@
+"""Compare forensic snapshots and print diff tables."""
+import json
+import sys
+from pathlib import Path
+
+def load(p):
+    return json.loads(Path(p).read_text(encoding="utf-8-sig"))
+
+def diff_table(a, b, keys):
+    rows = []
+    for k in keys:
+        va, vb = a.get(k), b.get(k)
+        changed = va != vb
+        rows.append((k, va, vb, "CHANGED" if changed else "same"))
+    return rows
+
+def print_table(title, rows):
+    print(f"\n=== {title} ===")
+    print(f"{'Property':<22} {'Before':<40} {'After':<40} {'Status'}")
+    for k, va, vb, st in rows:
+        sa = json.dumps(va) if not isinstance(va, str) else va
+        sb = json.dumps(vb) if not isinstance(vb, str) else vb
+        if len(sa) > 38: sa = sa[:35] + "..."
+        if len(sb) > 38: sb = sb[:35] + "..."
+        print(f"{k:<22} {sa:<40} {sb:<40} {st}")
+
+def compare_advances(orig, new):
+    oa, na = orig or [], new or []
+    diffs = []
+    for i in range(max(len(oa), len(na))):
+        ov = oa[i] if i < len(oa) else None
+        nv = na[i] if i < len(na) else None
+        if ov != nv:
+            diffs.append((i, ov, nv))
+    return diffs
+
+def main():
+    extract_path = Path(sys.argv[1] if len(sys.argv) > 1 else "forensic_extract_out.json")
+    wasm_path = Path(sys.argv[2] if len(sys.argv) > 2 else "forensic_wasm_out.json")
+    ex = load(extract_path)
+    wasm = load(wasm_path)
+
+    keys = [
+        "text", "fontId", "fontName", "fontSize", "color",
+        "glyphCount", "advanceCount", "paragraphWidth", "baseline", "lineHeight",
+        "origLinesProvided",
+    ]
+
+    orig = ex["original"]
+    entry_payload = ex["editEntry"]
+    one_payload = ex["editOne"]
+
+  # WASM layout snapshots
+    from forensic_extract import snapshot_from_wasm_layout, para_signature  # type: ignore
+    seed_fn = orig.get("fontName", "")
+    entry_wasm = snapshot_from_wasm_layout("ENTRY_WASM", wasm["editEntryWasm"]["layout"], seed_fn)
+    one_wasm = snapshot_from_wasm_layout("ONE_WASM", wasm["editOneWasm"]["layout"], seed_fn)
+
+    print_table("ORIGINAL vs EDIT_MODE_ENTRY (payload)", diff_table(orig, entry_payload, keys))
+    print_table("ORIGINAL vs EDIT_MODE_ENTRY (WASM layout)", diff_table(orig, entry_wasm, keys))
+
+    adv_diff_entry = compare_advances(orig.get("advances"), entry_wasm.get("advances"))
+    if adv_diff_entry:
+        print(f"\nAdvance diffs ORIGINAL vs ENTRY_WASM: {len(adv_diff_entry)} positions")
+        for i, ov, nv in adv_diff_entry[:20]:
+            print(f"  [{i}] {ov} -> {nv}")
+    else:
+        print("\nAdvance arrays IDENTICAL: ORIGINAL vs ENTRY_WASM")
+
+    print_table("EDIT_ENTRY vs AFTER_ONE_CHAR (payload)", diff_table(entry_payload, one_payload, keys))
+    print_table("ENTRY_WASM vs ONE_WASM", diff_table(entry_wasm, one_wasm, keys))
+
+    adv_diff_one = compare_advances(entry_wasm.get("advances"), one_wasm.get("advances"))
+    if adv_diff_one:
+        print(f"\nAdvance diffs ENTRY_WASM vs ONE_WASM: {len(adv_diff_one)} positions")
+        for i, ov, nv in adv_diff_one[:20]:
+            print(f"  [{i}] {ov} -> {nv}")
+
+    # Line widths
+    print("\n=== LINE WIDTHS ===")
+    print(f"Original lineWidths: {orig.get('lineWidths')}")
+    print(f"Entry WASM lineWidths: {entry_wasm.get('lineWidths')}")
+    print(f"One WASM lineWidths: {one_wasm.get('lineWidths')}")
+
+    # Signatures chain
+    sigs = {
+        "original": para_signature(orig),
+        "entry_payload": para_signature(entry_payload),
+        "entry_wasm": para_signature(entry_wasm),
+        "one_payload": para_signature(one_payload),
+        "one_wasm": para_signature(one_wasm),
+    }
+    print("\n=== SIGNATURE CHAIN ===")
+    prev = None
+    for name, sig in sigs.items():
+        changed = prev and sig != prev
+        print(f"  {name}: {sig}" + ("  <-- FIRST CHANGE from " + prev_name if changed else ""))
+        if changed and prev:
+            pass
+        prev_name = name
+        prev = sig
+
+    # First mutation
+    chain = list(sigs.items())
+    base = chain[0][1]
+    for i in range(1, len(chain)):
+        if chain[i][1] != base:
+            print(f"\nFIRST SIGNATURE CHANGE: {chain[i][0]} (vs original)")
+            break
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/edits/forensic_extract.py b/tests/edits/forensic_extract.py
new file mode 100644
index 0000000..fa05c04
--- /dev/null
+++ b/tests/edits/forensic_extract.py
@@ -0,0 +1,407 @@
+"""Stage 1-4 extraction + frontend layout mirror (ParagraphEditor.tsx)."""
+from __future__ import annotations
+
+import hashlib
+import json
+import sys
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+sys.path.insert(0, str(ROOT / "gateway"))
+import pdfengine  # type: ignore
+
+
+def line_advances(line) -> tuple[dict[int, list[float]], float]:
+    runs = line.runs
+    seq: list[tuple[int, int, float]] = []
+    aligned: dict[int, bool] = {}
+    for ri, r in enumerate(runs):
+        gs = r.glyphs
+        ok = bool(r.text) and len(gs) == len(r.text)
+        aligned[ri] = ok
+        if ok:
+            for ci, g in enumerate(gs):
+                seq.append((ri, ci, g.origin_x))
+    per_run: dict[int, list[float]] = {}
+    for ri, r in enumerate(runs):
+        if aligned.get(ri):
+            per_run[ri] = [0.0] * len(r.text)
+    anchor_x = seq[0][2] if seq else line.x
+    for k, (ri, ci, ox) in enumerate(seq):
+        gs = runs[ri].glyphs
+        g = gs[ci]
+        char_w = g.bbox_w if g.bbox_w > 0 else (runs[ri].font_size or 12) * 0.5
+        per_run[ri][ci] = (seq[k + 1][2] - ox) if k + 1 < len(seq) else char_w
+    for ri in list(per_run.keys()):
+        if any(a <= 0 for a in per_run[ri]):
+            del per_run[ri]
+    return per_run, anchor_x
+
+
+def para_eff_size(lines) -> float:
+    nominal = 0.0
+    for line in lines:
+        for r in line.runs:
+            sz = max(r.font_size or 0, r.h or 0)
+            if sz > nominal:
+                nominal = sz
+    return nominal or 12.0
+
+
+def median(xs: list[float]) -> float:
+    if not xs:
+        return 0.0
+    s = sorted(xs)
+    return s[len(s) // 2]
+
+
+def compute_layout(para) -> dict:
+    lines = para.lines
+    eff_size = para_eff_size(lines)
+    column_left = float("inf")
+    first_baseline_y = float("-inf")
+    baselines: list[float] = []
+    right_edges: list[float] = []
+    object_indices: list[int] = []
+    seed_runs: list[dict] = []
+    orig_lines: list[dict] = []
+
+    for li, line in enumerate(lines):
+        if line.baseline_y is not None:
+            baselines.append(line.baseline_y)
+            first_baseline_y = max(first_baseline_y, line.baseline_y)
+        column_left = min(column_left, line.x)
+        right_edges.append(line.x + line.w)
+        line_runs = line.runs
+        per_run, anchor_x = line_advances(line)
+        line_frags: list[dict] = []
+        for ri, r in enumerate(line_runs):
+            for oi in (r.object_indices or []):
+                object_indices.append(oi)
+            orig = r.text or ""
+            text = orig
+            if li > 0 and ri == 0 and seed_runs:
+                prev = seed_runs[-1]["text"]
+                if prev and not prev.endswith((" ", "\t")) and not text.startswith((" ", "\t")):
+                    text = " " + text
+            adv = per_run.get(ri)
+            fc = getattr(r, "fill_color", None) or getattr(r, "color", None) or "#000000"
+            safe_color = fc if fc and fc != "#ffffff00" else "#000000"
+            r_size = max(r.font_size or 0, r.h or 0) or eff_size
+            seed_runs.append({
+                "text": text, "fid": r.internal_font_id or "", "size": r_size,
+                "color": safe_color, "fontName": r.font_name or "", "advances": adv,
+            })
+            if orig:
+                line_frags.append({
+                    "text": orig, "fid": r.internal_font_id or "", "size": r_size,
+                    "color": safe_color, "advances": adv,
+                })
+        if line_frags:
+            orig_lines.append({"frags": line_frags, "x": anchor_x, "baselineY": line.baseline_y or 0})
+
+    max_right = max(right_edges) if right_edges else column_left + 250
+    column_right = max(max_right, column_left + 250) if column_left != float("inf") else 250
+    deltas = [baselines[i] - baselines[i + 1] for i in range(len(baselines) - 1)]
+    dom_size = next((r["size"] for r in seed_runs if r["text"].strip()), 12.0)
+    leading = abs(median(deltas)) if deltas else dom_size * 1.2
+    col_w = column_right - column_left
+    align = "left"
+    if len(lines) >= 2:
+        reaching = sum(1 for i in range(len(right_edges) - 1) if right_edges[i] >= column_right - col_w * 0.04)
+        if reaching >= (len(lines) - 1) * 0.7:
+            align = "justify"
+
+    return {
+        "columnLeft": column_left, "columnRight": column_right,
+        "firstBaselineY": first_baseline_y, "leading": leading,
+        "oldLineCount": len(lines), "align": align,
+        "objectIndices": object_indices, "seedRuns": seed_runs, "origLines": orig_lines,
+        "domSize": dom_size,
+    }
+
+
+def extract_flat_runs(seed_runs: list[dict], dominant_fid: str, dom_size: float, dom_color: str) -> list[dict]:
+    """Mirror extractFlatRuns when DOM matches seedRuns exactly (edit mode entry)."""
+    out: list[dict] = []
+    for r in seed_runs:
+        if not r["text"]:
+            continue
+        frag = {
+            "text": r["text"], "internalFontId": r["fid"] or dominant_fid,
+            "fontSize": r["size"], "color": r["color"],
+        }
+        adv = r.get("advances")
+        if adv and len(adv) == len(r["text"]):
+            frag["advances"] = adv
+        out.append(frag)
+    return out
+
+
+def build_reflow_data(layout: dict, runs: list[dict], orig_lines: list[dict] | None, para_id: str) -> dict:
+    line_position = {}
+    if layout["origLines"]:
+        line_position = {
+            "lineX": [l["x"] for l in layout["origLines"]],
+            "lineBaselineY": [l["baselineY"] for l in layout["origLines"]],
+        }
+    lines_data = {}
+    if orig_lines:
+        lines_data = {
+            "lines": [
+                [
+                    {
+                        "text": f["text"], "internalFontId": f["fid"],
+                        "fontSize": f["size"], "color": f["color"],
+                        **({"advances": f["advances"]} if f.get("advances") else {}),
+                    }
+                    for f in l["frags"]
+                ]
+                for l in orig_lines
+            ],
+        }
+    dominant_fid = next((r["fid"] for r in layout["seedRuns"] if r["text"].strip() and r["fid"]), "")
+    dom_run = next((r for r in layout["seedRuns"] if r["text"].strip() and r["fid"] == dominant_fid), None)
+    dom_size = (dom_run or layout["seedRuns"][0])["size"] if layout["seedRuns"] else 12.0
+    dom_color = (dom_run or layout["seedRuns"][0])["color"] if layout["seedRuns"] else "#000000"
+    out_runs = runs if runs else [{"text": " ", "internalFontId": dominant_fid, "fontSize": dom_size, "color": "#000000"}]
+    return {
+        "objectIndices": layout["objectIndices"],
+        "runs": out_runs,
+        **line_position,
+        **lines_data,
+        "columnLeft": layout["columnLeft"],
+        "columnRight": layout["columnRight"],
+        "pushColumnLeft": layout["columnLeft"],
+        "firstBaselineY": layout["firstBaselineY"],
+        "leading": layout["leading"],
+        "oldLineCount": layout["oldLineCount"],
+        "align": layout["align"],
+        "paraId": para_id,
+    }
+
+
+def paragraph_snapshot_from_extraction(para, layout: dict) -> dict:
+    """Original PDF paragraph metrics from extraction + derived advances."""
+    lines = para.lines
+    all_text = "".join(r.text for ln in lines for r in ln.runs)
+    runs_detail = []
+    for ln in lines:
+        per_run, anchor_x = line_advances(ln)
+        for ri, r in enumerate(ln.runs):
+            adv = per_run.get(ri)
+            runs_detail.append({
+                "text": r.text,
+                "fontId": r.internal_font_id,
+                "fontName": r.font_name,
+                "fontSize": max(r.font_size or 0, r.h or 0),
+                "color": getattr(r, "fill_color", None) or getattr(r, "color", None) or "#000000",
+                "glyphCount": len(r.glyphs),
+                "advanceCount": len(adv) if adv else 0,
+                "advances": adv,
+                "bbox": [r.x, r.y, r.w, r.h],
+                "lineWidth": sum(adv) if adv else r.w,
+            })
+    line_widths = [ln.w for ln in lines]
+    orig_line_texts = ["".join(r.text for r in ln.runs) for ln in lines]
+    return {
+        "stage": "ORIGINAL_PDF_EXTRACTION",
+        "text": all_text,
+        "fontId": runs_detail[0]["fontId"] if runs_detail else "",
+        "fontName": runs_detail[0]["fontName"] if runs_detail else "",
+        "fontSize": layout["domSize"],
+        "color": runs_detail[0]["color"] if runs_detail else "#000000",
+        "glyphCount": sum(len(r.glyphs) for ln in lines for r in ln.runs),
+        "advanceCount": sum(rd["advanceCount"] for rd in runs_detail),
+        "advances": [a for rd in runs_detail for a in (rd["advances"] or [])],
+        "boundingBox": [para.x, para.y, para.w, para.h],
+        "paragraphWidth": layout["columnRight"] - layout["columnLeft"],
+        "lineWidths": line_widths,
+        "baseline": layout["firstBaselineY"],
+        "ascent": None,
+        "descent": None,
+        "lineHeight": layout["leading"],
+        "origLines": orig_line_texts,
+        "runs": runs_detail,
+        "origLinesProvided": False,
+    }
+
+
+def snapshot_from_payload(stage: str, layout: dict, runs: list[dict], orig_lines: list[dict] | None) -> dict:
+    all_text = "".join(r["text"] for r in runs)
+    dom = runs[0] if runs else {}
+    adv_flat = []
+    for r in runs:
+        if r.get("advances"):
+            adv_flat.extend(r["advances"])
+    return {
+        "stage": stage,
+        "text": all_text,
+        "fontId": dom.get("internalFontId", ""),
+        "fontName": layout["seedRuns"][0]["fontName"] if layout["seedRuns"] else "",
+        "fontSize": dom.get("fontSize", layout["domSize"]),
+        "color": dom.get("color", "#000000"),
+        "glyphCount": len(all_text),
+        "advanceCount": len(adv_flat),
+        "advances": adv_flat,
+        "boundingBox": None,
+        "paragraphWidth": layout["columnRight"] - layout["columnLeft"],
+        "lineWidths": None,
+        "baseline": layout["firstBaselineY"],
+        "ascent": None,
+        "descent": None,
+        "lineHeight": layout["leading"],
+        "origLines": ["".join(f["text"] for f in l["frags"]) for l in orig_lines] if orig_lines else None,
+        "runs": [
+            {
+                "text": r["text"],
+                "fontId": r.get("internalFontId"),
+                "fontSize": r.get("fontSize"),
+                "color": r.get("color"),
+                "advanceCount": len(r.get("advances") or []),
+                "advances": r.get("advances"),
+                "advanceSource": "frontend_origin_x_derived" if r.get("advances") else "missing",
+            }
+            for r in runs
+        ],
+        "origLinesProvided": orig_lines is not None,
+    }
+
+
+def snapshot_from_wasm_layout(stage: str, layout_json: dict, seed_font_name: str) -> dict:
+    lines = layout_json.get("lines") or []
+    all_text = "".join(ln.get("text", "") for ln in lines)
+    adv_flat = []
+    line_widths = []
+    for ln in lines:
+        adv = ln.get("adv") or []
+        adv_flat.extend(adv)
+        line_widths.append(sum(adv))
+    dom_fs = lines[0].get("fontSize") if lines else None
+    return {
+        "stage": stage,
+        "text": all_text,
+        "fontId": "",
+        "fontName": seed_font_name,
+        "fontSize": dom_fs,
+        "color": None,
+        "glyphCount": len(all_text),
+        "advanceCount": len(adv_flat),
+        "advances": adv_flat,
+        "boundingBox": None,
+        "paragraphWidth": (layout_json.get("columnRight") or 0) - (layout_json.get("columnLeft") or 0),
+        "lineWidths": line_widths,
+        "baseline": lines[0].get("baselineY") if lines else None,
+        "ascent": None,
+        "descent": None,
+        "lineHeight": None,
+        "origLines": [ln.get("text", "") for ln in lines],
+        "runs": [
+            {
+                "text": ln.get("text"),
+                "lineX0": ln.get("x0"),
+                "fontSize": ln.get("fontSize"),
+                "advanceCount": len(ln.get("adv") or []),
+                "advances": ln.get("adv"),
+                "advanceSum": sum(ln.get("adv") or []),
+                "advanceSource": "wasm_reflow_output",
+            }
+            for ln in lines
+        ],
+        "origLinesProvided": None,
+    }
+
+
+def para_signature(snap: dict) -> str:
+    parts = [
+        snap.get("text") or "",
+        str(snap.get("fontId") or ""),
+        str(snap.get("fontSize") or ""),
+        json.dumps(snap.get("advances") or []),
+        json.dumps(snap.get("lineWidths") or []),
+        str(snap.get("baseline") or ""),
+        json.dumps(snap.get("boundingBox") or []),
+    ]
+    return hashlib.sha256("".join(parts).encode()).hexdigest()
+
+
+def main():
+    pdf_path = ROOT / "corpus" / "basic" / "hello_world.pdf"
+    if len(sys.argv) > 1:
+        pdf_path = Path(sys.argv[1])
+    doc = pdfengine.PdfDocument.load_from_file(str(pdf_path), "")
+    model = doc.get_page(0).extract_document_model()
+    para = model.paragraphs[0]
+    layout = compute_layout(para)
+    fonts = doc.get_fonts(0, 0)
+
+    dominant_fid = next((r["fid"] for r in layout["seedRuns"] if r["text"].strip() and r["fid"]), "")
+    dom_run = next((r for r in layout["seedRuns"] if r["text"].strip()), layout["seedRuns"][0])
+    dom_size = dom_run["size"]
+    dom_color = dom_run["color"]
+
+    para_id = f"forensic-{hashlib.md5(str(pdf_path).encode()).hexdigest()[:8]}"
+
+    # Stage snapshots
+    original = paragraph_snapshot_from_extraction(para, layout)
+
+    flat_entry = extract_flat_runs(layout["seedRuns"], dominant_fid, dom_size, dom_color)
+    payload_entry = build_reflow_data(layout, flat_entry, layout["origLines"], para_id)
+    edit_entry = snapshot_from_payload("EDIT_MODE_ENTRY_PAYLOAD", layout, flat_entry, layout["origLines"])
+
+    # After one character
+    flat_one = []
+    for i, r in enumerate(flat_entry):
+        if i == 0:
+            flat_one.append({**r, "text": r["text"] + "x"})
+        else:
+            flat_one.append(r)
+    payload_one = build_reflow_data(layout, flat_one, None, para_id)
+    edit_one = snapshot_from_payload("AFTER_ONE_CHAR_PAYLOAD", layout, flat_one, None)
+
+    out = {
+        "pdf": str(pdf_path),
+        "fonts": [
+            {
+                "internalFontId": f.internal_font_id,
+                "fontName": f.font_name,
+                "ascent": f.ascent,
+                "descent": f.descent,
+                "capHeight": getattr(f, "cap_height", None),
+                "unitsPerEm": getattr(f, "units_per_em", None),
+            }
+            for f in fonts
+        ],
+        "layout": layout,
+        "original": original,
+        "editEntry": edit_entry,
+        "editOne": edit_one,
+        "payloads": {
+            "editEntry": {
+                "version": "1.0",
+                "operations": [{
+                    "id": "forensic-entry", "type": "reflow_paragraph", "pageIndex": 0,
+                    "data": payload_entry,
+                }],
+            },
+            "editOne": {
+                "version": "1.0",
+                "operations": [{
+                    "id": "forensic-one", "type": "reflow_paragraph", "pageIndex": 0,
+                    "data": payload_one,
+                }],
+            },
+        },
+        "signatures": {
+            "original": para_signature(original),
+            "editEntryPayload": para_signature(edit_entry),
+            "editOnePayload": para_signature(edit_one),
+        },
+    }
+    out_path = Path(__file__).resolve().parent / "forensic_extract_out.json"
+    out_path.write_text(json.dumps(out, indent=2), encoding="utf-8")
+    print(json.dumps(out, indent=2))
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/edits/forensic_extract_out.json b/tests/edits/forensic_extract_out.json
new file mode 100644
index 0000000..3c2c8eb
--- /dev/null
+++ b/tests/edits/forensic_extract_out.json
@@ -0,0 +1,1178 @@
+{
+  "pdf": "tests\\edits\\forensic_multiline.pdf",
+  "fonts": [
+    {
+      "internalFontId": "Helvetica_Type1_32",
+      "fontName": "Helvetica",
+      "ascent": 905.0,
+      "descent": -211.0,
+      "capHeight": 728.0,
+      "unitsPerEm": null
+    }
+  ],
+  "layout": {
+    "columnLeft": 72.25299835205078,
+    "columnRight": 322.2529983520508,
+    "firstBaselineY": 700.0,
+    "leading": 14.0,
+    "oldLineCount": 3,
+    "align": "left",
+    "objectIndices": [
+      0,
+      1,
+      2
+    ],
+    "seedRuns": [
+      {
+        "text": "The quick brown fox jumps over the lazy",
+        "fid": "Helvetica_Type1_32",
+        "size": 11.0,
+        "color": "#000000",
+        "fontName": "Helvetica",
+        "advances": [
+          6.721000671386719,
+          6.115997314453125,
+          6.116004943847656,
+          3.0579986572265625,
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+          5.5,
+          5.22503662109375
+        ]
+      },
+      {
+        "text": " dog near the river bank on a sunny",
+        "fid": "Helvetica_Type1_32",
+        "size": 11.0,
+        "color": "#000000",
+        "fontName": "Helvetica",
+        "advances": [
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+          5.225006103515625
+        ]
+      },
+      {
+        "text": " afternoon in early spring.",
+        "fid": "Helvetica_Type1_32",
+        "size": 11.0,
+        "color": "#000000",
+        "fontName": "Helvetica",
+        "advances": [
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+      }
+    ],
+    "origLines": [
+      {
+        "frags": [
+          {
+            "text": "The quick brown fox jumps over the lazy",
+            "fid": "Helvetica_Type1_32",
+            "size": 11.0,
+            "color": "#000000",
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+          }
+        ],
+        "x": 72.0,
+        "baselineY": 700.0
+      },
+      {
+        "frags": [
+          {
+            "text": "dog near the river bank on a sunny",
+            "fid": "Helvetica_Type1_32",
+            "size": 11.0,
+            "color": "#000000",
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+        ],
+        "x": 72.0,
+        "baselineY": 686.0
+      },
+      {
+        "frags": [
+          {
+            "text": "afternoon in early spring.",
+            "fid": "Helvetica_Type1_32",
+            "size": 11.0,
+            "color": "#000000",
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+        ],
+        "x": 72.0,
+        "baselineY": 672.0
+      }
+    ],
+    "domSize": 11.0
+  },
+  "original": {
+    "stage": "ORIGINAL_PDF_EXTRACTION",
+    "text": "The quick brown fox jumps over the lazydog near the river bank on a sunnyafternoon in early spring.",
+    "fontId": "Helvetica_Type1_32",
+    "fontName": "Helvetica",
+    "fontSize": 11.0,
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+    "paragraphWidth": 250.0,
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+    "baseline": 700.0,
+    "ascent": null,
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+    "lineHeight": 14.0,
+    "origLines": [
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+      "dog near the river bank on a sunny",
+      "afternoon in early spring."
+    ],
+    "runs": [
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+        "fontId": "Helvetica_Type1_32",
+        "fontName": "Helvetica",
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+}
\ No newline at end of file
diff --git a/tests/edits/forensic_force_whole.py b/tests/edits/forensic_force_whole.py
new file mode 100644
index 0000000..289e9fe
--- /dev/null
+++ b/tests/edits/forensic_force_whole.py
@@ -0,0 +1,138 @@
+"""Step 2 experiment: force runPerChar=false for UNCHANGED paragraph.
+
+Requires rebuilt pdfengine with forensicForceWholeRun support.
+Compares:
+  A) default entry (runPerChar as decided by diverges)
+  B) forensicForceWholeRun=true (runPerChar forced 0)
+  C) original PDF
+
+Emits object dumps + content streams + optional region bitmap SHA.
+"""
+from __future__ import annotations
+
+import hashlib
+import json
+import sys
+import zlib
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+sys.path.insert(0, str(ROOT / "gateway"))
+# Prefer freshly rebuilt engine when present (must be AFTER gateway insert so it wins).
+_winlocal = Path(r"C:\Users\Maskan\pdfeng-build\win-local\lib")
+if _winlocal.exists():
+    sys.path.insert(0, str(_winlocal))
+import pdfengine  # type: ignore
+print(f"[engine] {pdfengine.__file__}", flush=True)
+
+sys.path.insert(0, str(Path(__file__).resolve().parent))
+from forensic_extract import compute_layout, extract_flat_runs, build_reflow_data  # type: ignore
+
+
+def inflate_content(pdf_bytes: bytes) -> list[str]:
+    out = []
+    for m in re.finditer(rb"stream\r?\n(.*?)\r?\nendstream", pdf_bytes, re.DOTALL):
+        try:
+            out.append(zlib.decompress(m.group(1)).decode("latin-1", errors="replace"))
+        except Exception:
+            pass
+    return out
+
+
+def font_dicts(pdf_bytes: bytes) -> list[str]:
+    found = []
+    for m in re.finditer(rb"<<[^>]*?/Type\s*/Font[^>]*?>>", pdf_bytes, re.DOTALL):
+        found.append(m.group(0).decode("latin-1", errors="replace"))
+    # also compact form
+    if not found:
+        for m in re.finditer(rb"<</BaseFont/[^>]+>>", pdf_bytes):
+            found.append(m.group(0).decode("latin-1", errors="replace"))
+    return found
+
+
+def region_sha(doc, page_index=0, dpi=144, y_top=200, height=80) -> str:
+    page = doc.get_page(page_index)
+    w, h, raw = page.render_region_raw(dpi, y_top, height)
+    return hashlib.sha256(raw).hexdigest()[:16], w, h
+
+
+def main():
+    pdf_path = Path(sys.argv[1]) if len(sys.argv) > 1 else ROOT / "corpus" / "basic" / "hello_world.pdf"
+    out_dir = Path(__file__).resolve().parent
+
+    doc0 = pdfengine.PdfDocument.load_from_file(str(pdf_path), "")
+    model = doc0.get_page(0).extract_document_model()
+    para = model.paragraphs[0]
+    layout = compute_layout(para)
+    dominant_fid = next((r["fid"] for r in layout["seedRuns"] if r["text"].strip() and r["fid"]), "")
+    dom = next((r for r in layout["seedRuns"] if r["text"].strip()), layout["seedRuns"][0])
+    flat = extract_flat_runs(layout["seedRuns"], dominant_fid, dom["size"], dom["color"])
+    data = build_reflow_data(layout, flat, layout["origLines"], "forensic-force-whole")
+
+    variants = {
+        "A_default_entry": {**data},
+        "B_force_whole_run": {**data, "forensicForceWholeRun": True},
+    }
+
+    orig_bytes = pdf_path.read_bytes()
+    print("ORIGINAL fonts:", font_dicts(orig_bytes))
+    print("ORIGINAL streams:")
+    for s in inflate_content(orig_bytes):
+        print(s)
+    try:
+        sha, w, h = region_sha(doc0, y_top=120, height=40)
+        print(f"ORIGINAL region sha={sha} {w}x{h}")
+    except Exception as e:
+        print("ORIGINAL region render failed:", e)
+
+    results = {}
+    for name, d in variants.items():
+        op = {"version": "1.0", "operations": [{
+            "id": name, "type": "reflow_paragraph", "pageIndex": 0, "data": d,
+        }]}
+        print(f"\n===== {name} =====")
+        doc = pdfengine.PdfDocument.load_from_file(str(pdf_path), "")
+        doc.apply_edits(json.dumps(op))
+        out = doc.save_full()
+        path = out_dir / f"_forensic_{name}.pdf"
+        path.write_bytes(out)
+        print("fonts:", font_dicts(out))
+        streams = inflate_content(out)
+        for s in streams:
+            # only show streams that mention Goodbye / FXF2 / 100 Tm
+            if "100 Tm" in s or "Goodbye" in s or "476F6F64" in s:
+                print("--- content ---")
+                print(s[:2000])
+        # count TJ
+        tj_count = sum(s.count(" TJ") for s in streams)
+        print(f"TJ operator count across streams: {tj_count}")
+        try:
+            d2 = pdfengine.PdfDocument.load_from_memory(out, "")
+            sha, w, h = region_sha(d2, y_top=120, height=40)
+            print(f"region sha={sha} {w}x{h}")
+            results[name] = {"sha": sha, "tj": tj_count, "fonts": font_dicts(out)}
+        except Exception as e:
+            print("region render failed:", e)
+            results[name] = {"error": str(e), "tj": tj_count}
+
+    print("\n===== VERDICT =====")
+    if "A_default_entry" in results and "B_force_whole_run" in results:
+        a, b = results["A_default_entry"], results["B_force_whole_run"]
+        print(f"A (default runPerChar) TJ={a.get('tj')} sha={a.get('sha')}")
+        print(f"B (force whole run)    TJ={b.get('tj')} sha={b.get('sha')}")
+        if a.get("sha") and b.get("sha"):
+            if a["sha"] == b["sha"]:
+                print("RESULT: A and B IDENTICAL bitmaps → runPerChar does NOT change pixels")
+            else:
+                print("RESULT: A and B DIFFER → runPerChar DOES change pixels")
+        if a.get("tj") == 1 and b.get("tj") and b["tj"] < a.get("tj", 99):
+            print("NOTE: B emitted fewer TJ (whole-run path active)")
+        if a.get("tj", 0) > 5 and b.get("tj", 0) <= 3:
+            print("CONFIRMED: forensicForceWholeRun reduced emission to whole-run")
+        elif a.get("tj") == b.get("tj"):
+            print("WARNING: TJ counts equal — rebuild may not include forensicForceWholeRun yet")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/edits/forensic_hash_compare.py b/tests/edits/forensic_hash_compare.py
new file mode 100644
index 0000000..08de358
--- /dev/null
+++ b/tests/edits/forensic_hash_compare.py
@@ -0,0 +1,130 @@
+"""Compare full-page render hashes + PDF objects across corpus files."""
+from __future__ import annotations
+
+import hashlib
+import json
+import re
+import sys
+import zlib
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+sys.path.insert(0, str(ROOT / "gateway"))
+import pdfengine  # type: ignore
+
+sys.path.insert(0, str(Path(__file__).resolve().parent))
+from forensic_extract import compute_layout, extract_flat_runs, build_reflow_data  # type: ignore
+from forensic_obj_compare import apply_and_save  # type: ignore
+
+
+def inflate(pdf_bytes: bytes) -> list[str]:
+    out = []
+    for m in re.finditer(rb"stream\r?\n(.*?)\r?\nendstream", pdf_bytes, re.DOTALL):
+        try:
+            out.append(zlib.decompress(m.group(1)).decode("latin-1", "replace"))
+        except Exception:
+            pass
+    return out
+
+
+def font_dicts(b: bytes) -> list[str]:
+    return [m.group(0).decode("latin-1", "replace") for m in re.finditer(rb"<</BaseFont/[^>]+>>", b)]
+
+
+def sha_full(doc, dpi=144) -> tuple[str, int]:
+    img = doc.get_page(0).render(dpi)
+    png = bytes(img.data)
+    return hashlib.sha256(png).hexdigest()[:24], len(png)
+
+
+def run(pdf: Path):
+    print(f"\n######## {pdf}")
+    doc = pdfengine.PdfDocument.load_from_file(str(pdf), "")
+    s0, n0 = sha_full(doc)
+    print(f"ORIG  sha={s0} pngbytes={n0}")
+    model = doc.get_page(0).extract_document_model()
+    if not model.paragraphs:
+        print("no paragraphs")
+        return
+    para = model.paragraphs[0]
+    layout = compute_layout(para)
+    for f in doc.get_fonts(0, 0)[:6]:
+        print(f"  FontInfo name={f.font_name} emb={f.is_embedded} type={f.type} "
+              f"subset={getattr(f, 'is_subset', None)} id={f.internal_font_id}")
+    print("  seedRuns:", [(r["fontName"], r["fid"], round(r["size"], 2),
+                           len(r["advances"]) if r.get("advances") else 0)
+                          for r in layout["seedRuns"][:4]])
+
+    fid = next((r["fid"] for r in layout["seedRuns"] if r["text"].strip() and r["fid"]), "")
+    dom = next((r for r in layout["seedRuns"] if r["text"].strip()), layout["seedRuns"][0])
+    flat = extract_flat_runs(layout["seedRuns"], fid, dom["size"], dom["color"])
+
+    data = build_reflow_data(layout, flat, layout["origLines"], "cmp")
+    op = {"version": "1.0", "operations": [{
+        "id": "e", "type": "reflow_paragraph", "pageIndex": 0, "data": data,
+    }]}
+    outp = Path(__file__).resolve().parent / "_tmp_entry.pdf"
+    apply_and_save(pdf, op, outp)
+    d2 = pdfengine.PdfDocument.load_from_memory(outp.read_bytes(), "")
+    s1, n1 = sha_full(d2)
+    print(f"ENTRY sha={s1} pngbytes={n1} {'IDENTICAL' if s0 == s1 else '*** DIFFERENT ***'}")
+    b = outp.read_bytes()
+    print("  entry fonts:", font_dicts(b))
+    for s in inflate(b):
+        if "Tm" in s or "TJ" in s or "Tj" in s:
+            one_line = s.replace("\n", " | ")
+            print("  stream:", one_line[:700])
+            tj = s.count(" TJ")
+            print(f"  TJ count={tj}")
+            break
+
+    flat1 = [{**flat[0], "text": flat[0]["text"] + "x"}] + flat[1:]
+    data1 = build_reflow_data(layout, flat1, None, "cmp")
+    # also try force whole if supported
+    data1_force = {**build_reflow_data(layout, flat, layout["origLines"], "cmp"),
+                   "forensicForceWholeRun": True}
+    op1 = {"version": "1.0", "operations": [{
+        "id": "o", "type": "reflow_paragraph", "pageIndex": 0, "data": data1,
+    }]}
+    out1 = Path(__file__).resolve().parent / "_tmp_one.pdf"
+    apply_and_save(pdf, op1, out1)
+    d3 = pdfengine.PdfDocument.load_from_memory(out1.read_bytes(), "")
+    s2, n2 = sha_full(d3)
+    print(f"ONE   sha={s2} pngbytes={n2} vsORIG={'SAME' if s0 == s2 else 'DIFF'} vsENTRY={'SAME' if s1 == s2 else 'DIFF'}")
+
+    # force whole on unchanged
+    opf = {"version": "1.0", "operations": [{
+        "id": "f", "type": "reflow_paragraph", "pageIndex": 0, "data": data1_force,
+    }]}
+    outf = Path(__file__).resolve().parent / "_tmp_force.pdf"
+    apply_and_save(pdf, opf, outf)
+    df = pdfengine.PdfDocument.load_from_memory(outf.read_bytes(), "")
+    sf, nf = sha_full(df)
+    bf = outf.read_bytes()
+    tjf = sum(s.count(" TJ") for s in inflate(bf))
+    tje = sum(s.count(" TJ") for s in inflate(b))
+    print(f"FORCE sha={sf} pngbytes={nf} vsORIG={'SAME' if s0 == sf else 'DIFF'} "
+          f"vsENTRY={'SAME' if s1 == sf else 'DIFF'} TJ_entry={tje} TJ_force={tjf}")
+    if "forensicForceWholeRun" in json.dumps(opf):
+        if tje != tjf:
+            print("  => forensicForceWholeRun ACTIVE (TJ count changed)")
+        else:
+            print("  => forensicForceWholeRun NOT in binary yet (TJ unchanged) OR no-op")
+
+
+def main():
+    for p in [
+        ROOT / "corpus" / "basic" / "hello_world.pdf",
+        ROOT / "tests" / "edits" / "forensic_multiline.pdf",
+        ROOT / "corpus" / "fonts" / "embedded_truetype.pdf",
+        ROOT / "corpus" / "fonts" / "subset_font.pdf",
+    ]:
+        if p.exists():
+            try:
+                run(p)
+            except Exception as e:
+                print(f"FAILED {p}: {e}")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/edits/forensic_multiline.pdf b/tests/edits/forensic_multiline.pdf
new file mode 100644
index 0000000..b76ac2e
--- /dev/null
+++ b/tests/edits/forensic_multiline.pdf
@@ -0,0 +1,35 @@
+%PDF-1.7
+1 0 obj
+<< /Type /Catalog /Pages 2 0 R >>
+endobj
+2 0 obj
+<< /Type /Pages /Kids [3 0 R] /Count 1 >>
+endobj
+3 0 obj
+<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] /Resources << /Font << /F1 5 0 R >> >> /Contents 4 0 R >>
+endobj
+4 0 obj
+<< /Length 238 >>
+stream
+BT /F1 11 Tf 72 700 Td (The quick brown fox jumps over the lazy) Tj ET
+BT /F1 11 Tf 72 686 Td (dog near the river bank on a sunny) Tj ET
+BT /F1 11 Tf 72 672 Td (afternoon in early spring.) Tj ET
+BT /F1 11 Tf 72 600 Td (FOOTER LINE) Tj ET
+endstream
+endobj
+5 0 obj
+<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica >>
+endobj
+xref
+0 6
+0000000000 65535 f 
+0000000009 00000 n 
+0000000058 00000 n 
+0000000115 00000 n 
+0000000241 00000 n 
+0000000529 00000 n 
+trailer
+<< /Size 6 /Root 1 0 R >>
+startxref
+599
+%%EOF
diff --git a/tests/edits/forensic_obj_compare.json b/tests/edits/forensic_obj_compare.json
new file mode 100644
index 0000000..0b8b3a9
--- /dev/null
+++ b/tests/edits/forensic_obj_compare.json
@@ -0,0 +1,849 @@
+{
+  "pdf": "corpus\\basic\\hello_world.pdf",
+  "fonts": [
+    {
+      "id": "Helvetica_Type1_32",
+      "name": "Helvetica",
+      "embedded": false,
+      "type": "Type1",
+      "subset": false,
+      "ascent": 905.0,
+      "descent": -211.0
+    },
+    {
+      "id": "Times-Roman_Type1_32",
+      "name": "Times-Roman",
+      "embedded": false,
+      "type": "Type1",
+      "subset": false,
+      "ascent": 891.0,
+      "descent": -215.0
+    }
+  ],
+  "orig": [
+    {
+      "para": 0,
+      "line": 0,
+      "run": 0,
+      "text": "Goodbye, world!",
+      "fontName": "Helvetica",
+      "fontId": "Helvetica_Type1_32",
+      "fontSize": 16.0,
+      "isEmbedded": false,
+      "fontType": "Type1",
+      "fontFidelity": "exact",
+      "x": 20.847999572753906,
+      "y": 96.65599822998047,
+      "w": 114.31999969482422,
+      "h": 14.99200439453125,
+      "objectIndices": [
+        1
+      ],
+      "glyphCount": 15,
+      "glyphOrigins": [
+        [
+          20.0,
+          100.0
+        ],
+        [
+          32.448,
+          100.0
+        ],
+        [
+          41.344,
+          100.0
+        ],
+        [
+          50.24,
+          100.0
+        ],
+        [
+          59.136,
+          100.0
+        ],
+        [
+          68.032,
+          100.0
+        ],
+        [
+          76.032,
+          100.0
+        ],
+        [
+          84.928,
+          100.0
+        ],
+        [
+          89.376,
+          100.0
+        ],
+        [
+          93.824,
+          100.0
+        ],
+        [
+          105.376,
+          100.0
+        ],
+        [
+          114.272,
+          100.0
+        ],
+        [
+          119.6,
+          100.0
+        ],
+        [
+          123.152,
+          100.0
+        ],
+        [
+          132.048,
+          100.0
+        ]
+      ],
+      "glyphUnicodes": [
+        71,
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+        111,
+        100,
+        98,
+        121,
+        101,
+        44,
+        32,
+        119,
+        111,
+        114,
+        108,
+        100,
+        33
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+      "fontMeta": {
+        "ascent": 905.0,
+        "descent": -211.0,
+        "capHeight": 728.0,
+        "isEmbedded": false,
+        "isSubset": false,
+        "type": "Type1",
+        "fontName": "Helvetica"
+      }
+    },
+    {
+      "para": 1,
+      "line": 0,
+      "run": 0,
+      "text": "Hello, world!",
+      "fontName": "Times-Roman",
+      "fontId": "Times-Roman_Type1_32",
+      "fontSize": 12.0,
+      "isEmbedded": false,
+      "fontType": "Type1",
+      "fontFidelity": "exact",
+      "x": 20.20400047302246,
+      "y": 48.007999420166016,
+      "w": 63.120004653930664,
+      "h": 10.319999694824219,
+      "objectIndices": [
+        0
+      ],
+      "glyphCount": 13,
+      "glyphOrigins": [
+        [
+          20.0,
+          50.0
+        ],
+        [
+          28.664,
+          50.0
+        ],
+        [
+          33.992,
+          50.0
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+        [
+          37.328,
+          50.0
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+        [
+          40.664,
+          50.0
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+        [
+          46.664,
+          50.0
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+        [
+          49.664,
+          50.0
+        ],
+        [
+          52.664,
+          50.0
+        ],
+        [
+          61.328,
+          50.0
+        ],
+        [
+          67.328,
+          50.0
+        ],
+        [
+          71.324,
+          50.0
+        ],
+        [
+          74.66,
+          50.0
+        ],
+        [
+          80.66,
+          50.0
+        ]
+      ],
+      "glyphUnicodes": [
+        72,
+        101,
+        108,
+        108,
+        111,
+        44,
+        32,
+        119,
+        111,
+        114,
+        108,
+        100,
+        33
+      ],
+      "fontMeta": {
+        "ascent": 891.0,
+        "descent": -215.0,
+        "capHeight": 662.0,
+        "isEmbedded": false,
+        "isSubset": false,
+        "type": "Type1",
+        "fontName": "Times-Roman"
+      }
+    }
+  ],
+  "entry": [
+    {
+      "para": 0,
+      "line": 0,
+      "run": 0,
+      "text": "Goodbye,",
+      "fontName": "Helvetica",
+      "fontId": "Helvetica_Type1_32",
+      "fontSize": 16.0,
+      "isEmbedded": false,
+      "fontType": "Type1",
+      "fontFidelity": "exact",
+      "x": 20.847999572753906,
+      "y": 96.65599822998047,
+      "w": 67.10400390625,
+      "h": 14.99200439453125,
+      "objectIndices": [
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+        13,
+        12,
+        11,
+        10,
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+        8,
+        7
+      ],
+      "glyphCount": 8,
+      "glyphOrigins": [
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+          20.0,
+          100.0
+        ],
+        [
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+          100.0
+        ],
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+          100.0
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+        ],
+        [
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+        ]
+      ],
+      "glyphUnicodes": [
+        71,
+        111,
+        111,
+        100,
+        98,
+        121,
+        101,
+        44
+      ],
+      "fontMeta": {
+        "ascent": 905.0,
+        "descent": -211.0,
+        "capHeight": 728.0,
+        "isEmbedded": false,
+        "isSubset": false,
+        "type": "Type1",
+        "fontName": "Helvetica"
+      }
+    },
+    {
+      "para": 0,
+      "line": 0,
+      "run": 1,
+      "text": " ",
+      "fontName": "",
+      "fontId": "",
+      "fontSize": 1.0,
+      "isEmbedded": false,
+      "fontType": "",
+      "fontFidelity": "exact",
+      "x": 89.3759994506836,
+      "y": 100.0,
+      "w": 0.0,
+      "h": 0.0,
+      "objectIndices": [],
+      "glyphCount": 1,
+      "glyphOrigins": [
+        [
+          89.376,
+          100.0
+        ]
+      ],
+      "glyphUnicodes": [
+        32
+      ],
+      "fontMeta": null
+    },
+    {
+      "para": 0,
+      "line": 0,
+      "run": 2,
+      "text": "world!",
+      "fontName": "Helvetica",
+      "fontId": "Helvetica_Type1_32",
+      "fontSize": 16.0,
+      "isEmbedded": false,
+      "fontType": "Type1",
+      "fontFidelity": "exact",
+      "x": 93.87199401855469,
+      "y": 99.8239974975586,
+      "w": 41.29600524902344,
+      "h": 11.632003784179688,
+      "objectIndices": [
+        6,
+        5,
+        4,
+        3,
+        2,
+        1
+      ],
+      "glyphCount": 6,
+      "glyphOrigins": [
+        [
+          93.824,
+          100.0
+        ],
+        [
+          105.376,
+          100.0
+        ],
+        [
+          114.272,
+          100.0
+        ],
+        [
+          119.6,
+          100.0
+        ],
+        [
+          123.152,
+          100.0
+        ],
+        [
+          132.048,
+          100.0
+        ]
+      ],
+      "glyphUnicodes": [
+        119,
+        111,
+        114,
+        108,
+        100,
+        33
+      ],
+      "fontMeta": {
+        "ascent": 905.0,
+        "descent": -211.0,
+        "capHeight": 728.0,
+        "isEmbedded": false,
+        "isSubset": false,
+        "type": "Type1",
+        "fontName": "Helvetica"
+      }
+    },
+    {
+      "para": 1,
+      "line": 0,
+      "run": 0,
+      "text": "Hello, world!",
+      "fontName": "Times-Roman",
+      "fontId": "Times-Roman_Type1_32",
+      "fontSize": 12.0,
+      "isEmbedded": false,
+      "fontType": "Type1",
+      "fontFidelity": "exact",
+      "x": 20.20400047302246,
+      "y": 48.007999420166016,
+      "w": 63.120004653930664,
+      "h": 10.319999694824219,
+      "objectIndices": [
+        0
+      ],
+      "glyphCount": 13,
+      "glyphOrigins": [
+        [
+          20.0,
+          50.0
+        ],
+        [
+          28.664,
+          50.0
+        ],
+        [
+          33.992,
+          50.0
+        ],
+        [
+          37.328,
+          50.0
+        ],
+        [
+          40.664,
+          50.0
+        ],
+        [
+          46.664,
+          50.0
+        ],
+        [
+          49.664,
+          50.0
+        ],
+        [
+          52.664,
+          50.0
+        ],
+        [
+          61.328,
+          50.0
+        ],
+        [
+          67.328,
+          50.0
+        ],
+        [
+          71.324,
+          50.0
+        ],
+        [
+          74.66,
+          50.0
+        ],
+        [
+          80.66,
+          50.0
+        ]
+      ],
+      "glyphUnicodes": [
+        72,
+        101,
+        108,
+        108,
+        111,
+        44,
+        32,
+        119,
+        111,
+        114,
+        108,
+        100,
+        33
+      ],
+      "fontMeta": {
+        "ascent": 891.0,
+        "descent": -215.0,
+        "capHeight": 662.0,
+        "isEmbedded": false,
+        "isSubset": false,
+        "type": "Type1",
+        "fontName": "Times-Roman"
+      }
+    }
+  ],
+  "one": [
+    {
+      "para": 0,
+      "line": 0,
+      "run": 0,
+      "text": "Goodbye,",
+      "fontName": "Helvetica",
+      "fontId": "Helvetica_Type1_32",
+      "fontSize": 16.0,
+      "isEmbedded": false,
+      "fontType": "Type1",
+      "fontFidelity": "exact",
+      "x": 20.847999572753906,
+      "y": 96.65599822998047,
+      "w": 67.10400390625,
+      "h": 14.99200439453125,
+      "objectIndices": [
+        2
+      ],
+      "glyphCount": 8,
+      "glyphOrigins": [
+        [
+          20.0,
+          100.0
+        ],
+        [
+          32.448,
+          100.0
+        ],
+        [
+          41.344,
+          100.0
+        ],
+        [
+          50.24,
+          100.0
+        ],
+        [
+          59.136,
+          100.0
+        ],
+        [
+          68.032,
+          100.0
+        ],
+        [
+          76.032,
+          100.0
+        ],
+        [
+          84.928,
+          100.0
+        ]
+      ],
+      "glyphUnicodes": [
+        71,
+        111,
+        111,
+        100,
+        98,
+        121,
+        101,
+        44
+      ],
+      "fontMeta": {
+        "ascent": 905.0,
+        "descent": -211.0,
+        "capHeight": 728.0,
+        "isEmbedded": false,
+        "isSubset": false,
+        "type": "Type1",
+        "fontName": "Helvetica"
+      }
+    },
+    {
+      "para": 0,
+      "line": 0,
+      "run": 1,
+      "text": " ",
+      "fontName": "",
+      "fontId": "",
+      "fontSize": 1.0,
+      "isEmbedded": false,
+      "fontType": "",
+      "fontFidelity": "exact",
+      "x": 89.3759994506836,
+      "y": 100.0,
+      "w": 0.0,
+      "h": 0.0,
+      "objectIndices": [],
+      "glyphCount": 1,
+      "glyphOrigins": [
+        [
+          89.376,
+          100.0
+        ]
+      ],
+      "glyphUnicodes": [
+        32
+      ],
+      "fontMeta": null
+    },
+    {
+      "para": 0,
+      "line": 0,
+      "run": 2,
+      "text": "world!x",
+      "fontName": "Helvetica",
+      "fontId": "Helvetica_Type1_32",
+      "fontSize": 16.0,
+      "isEmbedded": false,
+      "fontType": "Type1",
+      "fontFidelity": "exact",
+      "x": 93.8762435913086,
+      "y": 99.8239974975586,
+      "w": 50.512001037597656,
+      "h": 11.632003784179688,
+      "objectIndices": [
+        1
+      ],
+      "glyphCount": 7,
+      "glyphOrigins": [
+        [
+          93.828,
+          100.0
+        ],
+        [
+          105.38,
+          100.0
+        ],
+        [
+          114.276,
+          100.0
+        ],
+        [
+          119.604,
+          100.0
+        ],
+        [
+          123.156,
+          100.0
+        ],
+        [
+          132.052,
+          100.0
+        ],
+        [
+          136.5,
+          100.0
+        ]
+      ],
+      "glyphUnicodes": [
+        119,
+        111,
+        114,
+        108,
+        100,
+        33,
+        120
+      ],
+      "fontMeta": {
+        "ascent": 905.0,
+        "descent": -211.0,
+        "capHeight": 728.0,
+        "isEmbedded": false,
+        "isSubset": false,
+        "type": "Type1",
+        "fontName": "Helvetica"
+      }
+    },
+    {
+      "para": 1,
+      "line": 0,
+      "run": 0,
+      "text": "Hello, world!",
+      "fontName": "Times-Roman",
+      "fontId": "Times-Roman_Type1_32",
+      "fontSize": 12.0,
+      "isEmbedded": false,
+      "fontType": "Type1",
+      "fontFidelity": "exact",
+      "x": 20.20400047302246,
+      "y": 48.007999420166016,
+      "w": 63.120004653930664,
+      "h": 10.319999694824219,
+      "objectIndices": [
+        0
+      ],
+      "glyphCount": 13,
+      "glyphOrigins": [
+        [
+          20.0,
+          50.0
+        ],
+        [
+          28.664,
+          50.0
+        ],
+        [
+          33.992,
+          50.0
+        ],
+        [
+          37.328,
+          50.0
+        ],
+        [
+          40.664,
+          50.0
+        ],
+        [
+          46.664,
+          50.0
+        ],
+        [
+          49.664,
+          50.0
+        ],
+        [
+          52.664,
+          50.0
+        ],
+        [
+          61.328,
+          50.0
+        ],
+        [
+          67.328,
+          50.0
+        ],
+        [
+          71.324,
+          50.0
+        ],
+        [
+          74.66,
+          50.0
+        ],
+        [
+          80.66,
+          50.0
+        ]
+      ],
+      "glyphUnicodes": [
+        72,
+        101,
+        108,
+        108,
+        111,
+        44,
+        32,
+        119,
+        111,
+        114,
+        108,
+        100,
+        33
+      ],
+      "fontMeta": {
+        "ascent": 891.0,
+        "descent": -215.0,
+        "capHeight": 662.0,
+        "isEmbedded": false,
+        "isSubset": false,
+        "type": "Type1",
+        "fontName": "Times-Roman"
+      }
+    }
+  ],
+  "streams": {
+    "orig": {
+      "textOps": [
+        {
+          "op": "Tf",
+          "fontRes": "F1",
+          "size": 12.0
+        },
+        {
+          "op": "Tf",
+          "fontRes": "F2",
+          "size": 16.0
+        },
+        {
+          "op": "Td",
+          "tx": 20.0,
+          "ty": 50.0
+        },
+        {
+          "op": "Td",
+          "tx": 0.0,
+          "ty": 50.0
+        },
+        {
+          "op": "Tj",
+          "raw": "(Hello, world!) Tj"
+        },
+        {
+          "op": "Tj",
+          "raw": "(Goodbye, world!) Tj"
+        }
+      ],
+      "fontDicts": [
+        {
+          "subtype": "Type1",
+          "baseFont": "Times-Roman"
+        },
+        {
+          "subtype": "Type1",
+          "baseFont": "Helvetica"
+        }
+      ],
+      "streamCount": 1
+    },
+    "entry": {
+      "textOps": [],
+      "fontDicts": [
+        {
+          "subtype": "?",
+          "baseFont": "Times-Roman"
+        },
+        {
+          "subtype": "?",
+          "baseFont": "Helvetica"
+        }
+      ],
+      "streamCount": 2
+    },
+    "one": {
+      "textOps": [],
+      "fontDicts": [
+        {
+          "subtype": "?",
+          "baseFont": "Times-Roman"
+        },
+        {
+          "subtype": "?",
+          "baseFont": "Helvetica"
+        }
+      ],
+      "streamCount": 2
+    }
+  }
+}
\ No newline at end of file
diff --git a/tests/edits/forensic_obj_compare.py b/tests/edits/forensic_obj_compare.py
new file mode 100644
index 0000000..e6d8ca9
--- /dev/null
+++ b/tests/edits/forensic_obj_compare.py
@@ -0,0 +1,308 @@
+"""Forensic: dump & compare PDF text objects — original vs edit-entry vs one-char.
+
+Does NOT compare images. Compares:
+  font base name, font size, matrix, bbox, unicode text, object count,
+  and raw content-stream snippets around Tj/TJ.
+
+Also answers Step 4 from static path + runtime FontInfo:
+  why measureFace is nullptr for this paragraph's font.
+"""
+from __future__ import annotations
+
+import json
+import re
+import sys
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+sys.path.insert(0, str(ROOT / "gateway"))
+import pdfengine  # type: ignore
+
+
+def dump_page_text_objects(doc, page_index: int = 0) -> list[dict]:
+    """Re-extract model and also pull flat glyph list as proxy for text objects."""
+    page = doc.get_page(page_index)
+    model = page.extract_document_model()
+    fonts = doc.get_fonts(page_index, page_index)
+    font_by_id = {f.internal_font_id: f for f in fonts}
+
+    objs = []
+    for pi, para in enumerate(model.paragraphs):
+        for li, line in enumerate(para.lines):
+            for ri, run in enumerate(line.runs):
+                fi = font_by_id.get(run.internal_font_id)
+                objs.append({
+                    "para": pi, "line": li, "run": ri,
+                    "text": run.text,
+                    "fontName": run.font_name,
+                    "fontId": run.internal_font_id,
+                    "fontSize": run.font_size,
+                    "isEmbedded": run.is_embedded,
+                    "fontType": run.type,
+                    "fontFidelity": getattr(run, "font_fidelity", None),
+                    "x": run.x, "y": run.y, "w": run.w, "h": run.h,
+                    "objectIndices": list(run.object_indices or []),
+                    "glyphCount": len(run.glyphs),
+                    "glyphOrigins": [(round(g.origin_x, 3), round(g.origin_y, 3)) for g in run.glyphs[:40]],
+                    "glyphUnicodes": [g.unicode for g in run.glyphs[:40]],
+                    "fontMeta": None if not fi else {
+                        "ascent": fi.ascent, "descent": fi.descent,
+                        "capHeight": getattr(fi, "cap_height", None),
+                        "isEmbedded": fi.is_embedded,
+                        "isSubset": getattr(fi, "is_subset", None),
+                        "type": fi.type,
+                        "fontName": fi.font_name,
+                    },
+                })
+    return objs
+
+
+def extract_content_stream_text_ops(pdf_bytes: bytes) -> dict:
+    """Best-effort parse of content stream for font/text operators."""
+    # Find stream ... endstream blocks
+    streams = re.findall(rb"stream\r?\n(.*?)\r?\nendstream", pdf_bytes, re.DOTALL)
+    ops = []
+    for s in streams:
+        # Decode latin-1 so binary stays 1:1
+        try:
+            text = s.decode("latin-1")
+        except Exception:
+            continue
+        # Font select: /F1 16 Tf or /Helv 11 Tf
+        for m in re.finditer(r"/([A-Za-z0-9\+\,\-]+)\s+([\d.]+)\s+Tf", text):
+            ops.append({"op": "Tf", "fontRes": m.group(1), "size": float(m.group(2))})
+        # Text matrix: a b c d e f Tm
+        for m in re.finditer(
+            r"([\d.\-]+)\s+([\d.\-]+)\s+([\d.\-]+)\s+([\d.\-]+)\s+([\d.\-]+)\s+([\d.\-]+)\s+Tm",
+            text,
+        ):
+            ops.append({"op": "Tm", "matrix": [float(x) for x in m.groups()]})
+        # Td
+        for m in re.finditer(r"([\d.\-]+)\s+([\d.\-]+)\s+Td", text):
+            ops.append({"op": "Td", "tx": float(m.group(1)), "ty": float(m.group(2))})
+        # Literal strings before Tj
+        for m in re.finditer(r"\((?:\\.|[^\\)])*\)\s*Tj", text):
+            raw = m.group(0)
+            ops.append({"op": "Tj", "raw": raw[:120]})
+        for m in re.finditer(r"\[.*?\]\s*TJ", text, re.DOTALL):
+            ops.append({"op": "TJ", "raw": m.group(0)[:200]})
+    # Font dictionaries
+    fonts = []
+    for m in re.finditer(
+        rb"/Type\s*/Font\s*/Subtype\s*/(\w+)\s*/BaseFont\s*/([^\s/]+)",
+        pdf_bytes,
+    ):
+        fonts.append({"subtype": m.group(1).decode(), "baseFont": m.group(2).decode()})
+    # Also looser BaseFont
+    if not fonts:
+        for m in re.finditer(rb"/BaseFont\s*/([^\s/]+)", pdf_bytes):
+            fonts.append({"subtype": "?", "baseFont": m.group(1).decode()})
+    return {"textOps": ops, "fontDicts": fonts, "streamCount": len(streams)}
+
+
+def load_layout(path: Path):
+    sys.path.insert(0, str(path.parent))
+    from forensic_extract import (  # type: ignore
+        compute_layout, extract_flat_runs, build_reflow_data,
+    )
+    return compute_layout, extract_flat_runs, build_reflow_data
+
+
+def build_payloads(pdf_path: Path):
+    compute_layout, extract_flat_runs, build_reflow_data = load_layout(
+        Path(__file__).resolve().parent
+    )
+    doc = pdfengine.PdfDocument.load_from_file(str(pdf_path), "")
+    model = doc.get_page(0).extract_document_model()
+    para = model.paragraphs[0]
+    layout = compute_layout(para)
+    dominant_fid = next(
+        (r["fid"] for r in layout["seedRuns"] if r["text"].strip() and r["fid"]), ""
+    )
+    dom = next((r for r in layout["seedRuns"] if r["text"].strip()), layout["seedRuns"][0])
+    flat = extract_flat_runs(layout["seedRuns"], dominant_fid, dom["size"], dom["color"])
+    para_id = "forensic-obj-cmp"
+
+    entry_data = build_reflow_data(layout, flat, layout["origLines"], para_id)
+    # one char: append x, drop origLines, keep stale advances (mirrors real editor)
+    flat_one = [{**flat[0], "text": flat[0]["text"] + "x"}] + flat[1:]
+    one_data = build_reflow_data(layout, flat_one, None, para_id)
+
+    return {
+        "entry": {"version": "1.0", "operations": [{
+            "id": "e", "type": "reflow_paragraph", "pageIndex": 0, "data": entry_data,
+        }]},
+        "one": {"version": "1.0", "operations": [{
+            "id": "o", "type": "reflow_paragraph", "pageIndex": 0, "data": one_data,
+        }]},
+        "layout": layout,
+        "fonts": [
+            {
+                "id": f.internal_font_id, "name": f.font_name,
+                "embedded": f.is_embedded, "type": f.type,
+                "subset": getattr(f, "is_subset", None),
+                "ascent": f.ascent, "descent": f.descent,
+            }
+            for f in doc.get_fonts(0, 0)
+        ],
+    }
+
+
+def apply_and_save(pdf_path: Path, op: dict, out_path: Path) -> bytes:
+    doc = pdfengine.PdfDocument.load_from_file(str(pdf_path), "")
+    doc.apply_edits(json.dumps(op))
+    data = doc.save_full()
+    out_path.write_bytes(data)
+    return data
+
+
+def compare_objs(label_a: str, a: list[dict], label_b: str, b: list[dict]):
+    print(f"\n{'='*60}\nCOMPARE {label_a} vs {label_b}\n{'='*60}")
+    print(f"  object/run count: {len(a)} -> {len(b)}")
+    keys = ["text", "fontName", "fontId", "fontSize", "isEmbedded", "fontType",
+            "w", "glyphCount"]
+    n = max(len(a), len(b))
+    for i in range(min(n, 12)):
+        oa = a[i] if i < len(a) else None
+        ob = b[i] if i < len(b) else None
+        if oa is None:
+            print(f"  [{i}] ADDED: {ob['text']!r} font={ob['fontName']} size={ob['fontSize']}")
+            continue
+        if ob is None:
+            print(f"  [{i}] REMOVED: {oa['text']!r}")
+            continue
+        diffs = []
+        for k in keys:
+            if oa.get(k) != ob.get(k):
+                diffs.append(f"{k}: {oa.get(k)!r} -> {ob.get(k)!r}")
+        # origin of first glyph
+        goa = oa["glyphOrigins"][0] if oa["glyphOrigins"] else None
+        gob = ob["glyphOrigins"][0] if ob["glyphOrigins"] else None
+        if goa != gob:
+            diffs.append(f"firstOrigin: {goa} -> {gob}")
+        status = "CHANGED" if diffs else "same"
+        print(f"  [{i}] {oa['text']!r:30} {status}")
+        for d in diffs:
+            print(f"       {d}")
+
+
+def explain_measureface(fonts: list[dict]):
+    print(f"\n{'='*60}\nSTEP 4 — why measureFace is nullptr\n{'='*60}")
+    for f in fonts:
+        print(f"  FontInfo: id={f['id']} name={f['name']} embedded={f['embedded']} "
+              f"type={f['type']} subset={f['subset']}")
+        if not f["embedded"]:
+            print("  PATH TAKEN in loadEmissionFont():")
+            print("    matchedFontInfo && !isEmbedded")
+            print("      -> cacheKey = \"standard_\" + fontName")
+            print("      -> useEmbedded = false")
+            print("      -> useSystem   = false")
+            print("    skip embedded/system load branches")
+            print("    fall through to FPDFText_LoadStandardFont()")
+            print("    measureFace ONLY set from loadedFontDataBuffers_ (TTF bytes)")
+            print("    standard fonts never put bytes in that buffer")
+            print("  => measureFace remains nullptr BY DESIGN for non-embedded fonts")
+        else:
+            print("  Embedded path: measureFace set only if getFontData*/LoadFromMemory succeeds")
+            print("  or via useSystem branch FONT_METRICS_FIX preserving orig face")
+
+
+def main():
+    pdf_path = Path(sys.argv[1]) if len(sys.argv) > 1 else ROOT / "corpus" / "basic" / "hello_world.pdf"
+    out_dir = Path(__file__).resolve().parent
+
+    print(f"PDF: {pdf_path}")
+    payloads = build_payloads(pdf_path)
+    explain_measureface(payloads["fonts"])
+
+    # Original
+    orig_doc = pdfengine.PdfDocument.load_from_file(str(pdf_path), "")
+    orig_objs = dump_page_text_objects(orig_doc)
+    orig_bytes = pdf_path.read_bytes()
+    orig_stream = extract_content_stream_text_ops(orig_bytes)
+
+    print(f"\n{'='*60}\nORIGINAL PDF TEXT OBJECTS\n{'='*60}")
+    for o in orig_objs:
+        print(f"  text={o['text']!r} font={o['fontName']} id={o['fontId']} "
+              f"size={o['fontSize']:.2f} emb={o['isEmbedded']} type={o['fontType']} "
+              f"w={o['w']:.3f} glyphs={o['glyphCount']} objs={o['objectIndices']}")
+        if o["fontMeta"]:
+            print(f"    meta: ascent={o['fontMeta']['ascent']} descent={o['fontMeta']['descent']} "
+                  f"subset={o['fontMeta']['isSubset']}")
+    print(f"  content fontDicts: {orig_stream['fontDicts']}")
+    print(f"  content Tf/Tj ops ({len(orig_stream['textOps'])}):")
+    for op in orig_stream["textOps"][:20]:
+        print(f"    {op}")
+
+    # Entry preview (unchanged text)
+    print(f"\n--- applying EDIT ENTRY reflow (unchanged text) ---")
+    entry_path = out_dir / "_forensic_entry.pdf"
+    entry_bytes = apply_and_save(pdf_path, payloads["entry"], entry_path)
+    entry_doc = pdfengine.PdfDocument.load_from_memory(entry_bytes, "")
+    entry_objs = dump_page_text_objects(entry_doc)
+    entry_stream = extract_content_stream_text_ops(entry_bytes)
+
+    print(f"\n{'='*60}\nEDIT-ENTRY PDF TEXT OBJECTS\n{'='*60}")
+    for o in entry_objs[:20]:
+        print(f"  text={o['text']!r} font={o['fontName']} id={o['fontId']} "
+              f"size={o['fontSize']:.2f} emb={o['isEmbedded']} type={o['fontType']} "
+              f"w={o['w']:.3f} glyphs={o['glyphCount']} objs={o['objectIndices']}")
+    print(f"  content fontDicts: {entry_stream['fontDicts']}")
+    print(f"  content Tf/Tj ops ({len(entry_stream['textOps'])}):")
+    for op in entry_stream["textOps"][:30]:
+        print(f"    {op}")
+
+    compare_objs("ORIGINAL", orig_objs, "EDIT-ENTRY", entry_objs)
+
+    # One char
+    print(f"\n--- applying ONE-CHAR reflow ---")
+    one_path = out_dir / "_forensic_one.pdf"
+    one_bytes = apply_and_save(pdf_path, payloads["one"], one_path)
+    one_doc = pdfengine.PdfDocument.load_from_memory(one_bytes, "")
+    one_objs = dump_page_text_objects(one_doc)
+    one_stream = extract_content_stream_text_ops(one_bytes)
+
+    print(f"\n{'='*60}\nONE-CHAR PDF TEXT OBJECTS\n{'='*60}")
+    for o in one_objs[:20]:
+        print(f"  text={o['text']!r} font={o['fontName']} id={o['fontId']} "
+              f"size={o['fontSize']:.2f} emb={o['isEmbedded']} type={o['fontType']} "
+              f"w={o['w']:.3f} glyphs={o['glyphCount']} objs={o['objectIndices']}")
+    print(f"  content fontDicts: {one_stream['fontDicts']}")
+    print(f"  content Tf/Tj ops ({len(one_stream['textOps'])}):")
+    for op in one_stream["textOps"][:30]:
+        print(f"    {op}")
+
+    compare_objs("EDIT-ENTRY", entry_objs, "ONE-CHAR", one_objs)
+    compare_objs("ORIGINAL", orig_objs, "ONE-CHAR", one_objs)
+
+    # Summarize first differences
+    print(f"\n{'='*60}\nFIRST DIFFERENCES SUMMARY\n{'='*60}")
+    print(f"  Original run count:     {len(orig_objs)}")
+    print(f"  Edit-entry run count:   {len(entry_objs)}")
+    print(f"  One-char run count:     {len(one_objs)}")
+    print(f"  Original Tj/TJ ops:     {sum(1 for o in orig_stream['textOps'] if o['op'] in ('Tj','TJ'))}")
+    print(f"  Entry Tj/TJ ops:        {sum(1 for o in entry_stream['textOps'] if o['op'] in ('Tj','TJ'))}")
+    print(f"  One-char Tj/TJ ops:     {sum(1 for o in one_stream['textOps'] if o['op'] in ('Tj','TJ'))}")
+    print(f"  Original fontDicts:     {orig_stream['fontDicts']}")
+    print(f"  Entry fontDicts:        {entry_stream['fontDicts']}")
+    print(f"  One-char fontDicts:     {one_stream['fontDicts']}")
+
+    report = {
+        "pdf": str(pdf_path),
+        "fonts": payloads["fonts"],
+        "orig": orig_objs,
+        "entry": entry_objs,
+        "one": one_objs,
+        "streams": {
+            "orig": orig_stream,
+            "entry": entry_stream,
+            "one": one_stream,
+        },
+    }
+    out_json = out_dir / "forensic_obj_compare.json"
+    out_json.write_text(json.dumps(report, indent=2), encoding="utf-8")
+    print(f"\nWrote {out_json}")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/edits/forensic_obj_compare_out.txt b/tests/edits/forensic_obj_compare_out.txt
new file mode 100644
index 0000000..1f82a1d
Binary files /dev/null and b/tests/edits/forensic_obj_compare_out.txt differ
diff --git a/tests/edits/forensic_pdf_objects.py b/tests/edits/forensic_pdf_objects.py
new file mode 100644
index 0000000..cfbc85d
--- /dev/null
+++ b/tests/edits/forensic_pdf_objects.py
@@ -0,0 +1,48 @@
+"""Post-reflow PDF object inspection via Python pdfengine."""
+import json
+import sys
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+sys.path.insert(0, str(ROOT / "gateway"))
+import pdfengine  # type: ignore
+
+
+def inspect_pdf(path: Path, label: str):
+    doc = pdfengine.PdfDocument.load_from_file(str(path), "")
+    model = doc.get_page(0).extract_document_model()
+    para = model.paragraphs[0]
+    print(f"\n=== {label} ({path.name}) ===")
+    for li, ln in enumerate(para.lines):
+        for ri, r in enumerate(ln.runs):
+            adv_sum = 0.0
+            if r.glyphs:
+                # derive advances from origin_x
+                gs = r.glyphs
+                for i in range(len(gs)):
+                    if i + 1 < len(gs):
+                        adv_sum += gs[i + 1].origin_x - gs[i].origin_x
+                    else:
+                        adv_sum += gs[i].bbox_w or r.font_size * 0.5
+            print(f"  line{li} run{ri}: text={repr(r.text[:40])} font={r.font_name} size={r.font_size:.3f} w={r.w:.3f} advSum={adv_sum:.3f} glyphs={len(r.glyphs)}")
+
+
+def main():
+    extract_path = ROOT / "tests" / "edits" / "forensic_extract_out.json"
+    ex = json.loads(extract_path.read_text(encoding="utf-8"))
+    pdf_path = Path(ex["pdf"])
+    inspect_pdf(pdf_path, "ORIGINAL")
+
+    doc = pdfengine.PdfDocument.load_from_file(str(pdf_path), "")
+    for label, key in [("ENTRY", "editEntry"), ("ONE_CHAR", "editOne")]:
+        op = ex["payloads"][key]
+        d = pdfengine.PdfDocument.load_from_file(str(pdf_path), "")
+        d.apply_edits(json.dumps(op))
+        out = d.save_full()
+        tmp = ROOT / "tests" / "edits" / f"_forensic_tmp_{key}.pdf"
+        tmp.write_bytes(out)
+        inspect_pdf(tmp, label)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/edits/forensic_wasm.mjs b/tests/edits/forensic_wasm.mjs
new file mode 100644
index 0000000..14e9db5
--- /dev/null
+++ b/tests/edits/forensic_wasm.mjs
@@ -0,0 +1,68 @@
+/**
+ * WASM reflow forensic: run payloads and capture lastLayoutJson.
+ * Usage: node forensic_wasm.mjs <extract.json> [mjsPath]
+ */
+import { readFileSync, writeFileSync, existsSync } from "node:fs";
+import { dirname, resolve } from "node:path";
+import { fileURLToPath, pathToFileURL } from "node:url";
+
+const here = dirname(fileURLToPath(import.meta.url));
+const repoRoot = resolve(here, "../../");
+
+const extractPath = process.argv[2] || resolve(here, "forensic_extract_out.json");
+const mjsPath = process.argv[3] || resolve(repoRoot, "frontend/public/pdfium-engine.mjs");
+
+if (!existsSync(extractPath)) {
+  console.error(`Missing extract JSON: ${extractPath}`);
+  process.exit(1);
+}
+if (!existsSync(mjsPath)) {
+  console.error(`Missing WASM module: ${mjsPath}`);
+  process.exit(1);
+}
+
+const extract = JSON.parse(readFileSync(extractPath, "utf8"));
+const pdfBytes = readFileSync(extract.pdf);
+
+const { default: createModule } = await import(pathToFileURL(mjsPath).href);
+const wasmPath = mjsPath.replace(/\.mjs$/, ".wasm");
+const Module = await createModule({
+  locateFile: (path) => (path.endsWith(".wasm") ? wasmPath : path),
+});
+
+const ptr = Module._malloc(pdfBytes.length);
+Module.HEAPU8.set(pdfBytes, ptr);
+const handle = Module.ccall("loadDocument", "number", ["number", "number"], [ptr, pdfBytes.length]);
+Module._free(ptr);
+if (handle < 0) {
+  console.error("loadDocument failed");
+  process.exit(1);
+}
+
+async function runReflow(label, opJson) {
+  const editsStr = JSON.stringify(opJson);
+  const len = Module.ccall(
+    "previewRender",
+    "number",
+    ["number", "number", "number", "string"],
+    [handle, 0, 144, editsStr],
+  );
+  const layoutJson = Module.ccall("lastLayoutJson", "string", [], []);
+  let layout = null;
+  try {
+    layout = JSON.parse(layoutJson || "{}");
+  } catch {
+    layout = { raw: layoutJson };
+  }
+  return { label, renderBytes: len, layout };
+}
+
+const results = {
+  editEntryWasm: await runReflow("EDIT_MODE_ENTRY_WASM", extract.payloads.editEntry),
+  editOneWasm: await runReflow("AFTER_ONE_CHAR_WASM", extract.payloads.editOne),
+};
+
+writeFileSync(resolve(here, "forensic_wasm_out.json"), JSON.stringify(results, null, 2));
+console.log(JSON.stringify(results, null, 2));
+
+Module.ccall("freeDocument", null, ["number"], [handle]);
diff --git a/tests/edits/forensic_wasm_embedded.json b/tests/edits/forensic_wasm_embedded.json
new file mode 100644
index 0000000..c1e6241
Binary files /dev/null and b/tests/edits/forensic_wasm_embedded.json differ
diff --git a/tests/edits/forensic_wasm_hello.json b/tests/edits/forensic_wasm_hello.json
new file mode 100644
index 0000000..7c08582
--- /dev/null
+++ b/tests/edits/forensic_wasm_hello.json
@@ -0,0 +1,71 @@
+{
+  "editEntryWasm": {
+    "label": "EDIT_MODE_ENTRY_WASM",
+    "renderBytes": 10936,
+    "layout": {
+      "anchorPage": 0,
+      "columnLeft": 20.847999572753906,
+      "lines": [
+        {
+          "adv": [
+            12.447998046875,
+            8.896003723144531,
+            8.89599609375,
+            8.896003723144531,
+            8.89599609375,
+            8,
+            8.896003723144531,
+            4.447998046875,
+            4.447998046875,
+            11.552001953125,
+            8.896003723144531,
+            5.3280029296875,
+            3.552001953125,
+            8.89599609375,
+            1.743988037109375
+          ],
+          "baselineY": 100,
+          "fontSize": 16,
+          "pageIndex": 0,
+          "text": "Goodbye, world!",
+          "x0": 20
+        }
+      ]
+    }
+  },
+  "editOneWasm": {
+    "label": "AFTER_ONE_CHAR_WASM",
+    "renderBytes": 11454,
+    "layout": {
+      "anchorPage": 0,
+      "columnLeft": 20.847999572753906,
+      "lines": [
+        {
+          "adv": [
+            10.09375,
+            8.4375,
+            8.4375,
+            8.40625,
+            8.32825,
+            7.047,
+            7.961,
+            3.9922500000000003,
+            3.61725,
+            11.289,
+            8.4375,
+            5.57825,
+            3.672,
+            8.40625,
+            5.211,
+            6.92975
+          ],
+          "baselineY": 100,
+          "fontSize": 16,
+          "pageIndex": 0,
+          "text": "Goodbye, world!x",
+          "x0": 20
+        }
+      ]
+    }
+  }
+}
diff --git a/tests/edits/forensic_wasm_multiline.json b/tests/edits/forensic_wasm_multiline.json
new file mode 100644
index 0000000..26133fb
--- /dev/null
+++ b/tests/edits/forensic_wasm_multiline.json
@@ -0,0 +1,267 @@
+{
+  "editEntryWasm": {
+    "label": "EDIT_MODE_ENTRY_WASM",
+    "renderBytes": 36617,
+    "layout": {
+      "anchorPage": 0,
+      "columnLeft": 72.25299835205078,
+      "lines": [
+        {
+          "adv": [
+            6.721000671386719,
+            6.115997314453125,
+            6.116004943847656,
+            3.0579986572265625,
+            6.115997314453125,
+            6.115997314453125,
+            2.4420013427734375,
+            5.5,
+            5.5,
+            3.0579986572265625,
+            6.1160125732421875,
+            3.662994384765625,
+            6.115997314453125,
+            7.9420013427734375,
+            6.115997314453125,
+            3.0579986572265625,
+            3.0579986572265625,
+            6.115997314453125,
+            5.5,
+            3.0579986572265625,
+            2.4420013427734375,
+            6.115997314453125,
+            9.163009643554688,
+            6.115997314453125,
+            5.5,
+            3.0579986572265625,
+            6.115997314453125,
+            5.5,
+            6.115997314453125,
+            3.662994384765625,
+            3.0579986572265625,
+            3.0579986572265625,
+            6.115997314453125,
+            6.115997314453125,
+            3.0579986572265625,
+            2.4420013427734375,
+            6.1160125732421875,
+            5.5,
+            5.22503662109375
+          ],
+          "baselineY": 700,
+          "fontSize": 11,
+          "pageIndex": 0,
+          "text": "The quick brown fox jumps over the lazy",
+          "x0": 72
+        },
+        {
+          "adv": [
+            6.115997314453125,
+            6.116004943847656,
+            6.115997314453125,
+            3.0579986572265625,
+            6.116004943847656,
+            6.115997314453125,
+            6.115997314453125,
+            3.6630020141601562,
+            3.0579986572265625,
+            3.0579986572265625,
+            6.116004943847656,
+            6.115997314453125,
+            3.0579986572265625,
+            3.662994384765625,
+            2.4420166015625,
+            5.5,
+            6.115997314453125,
+            3.662994384765625,
+            3.0579986572265625,
+            6.115997314453125,
+            6.115997314453125,
+            6.115997314453125,
+            5.5,
+            3.0579986572265625,
+            6.115997314453125,
+            6.115997314453125,
+            3.0579986572265625,
+            6.115997314453125,
+            3.0579986572265625,
+            5.5,
+            6.115997314453125,
+            6.115997314453125,
+            6.115997314453125,
+            5.225006103515625
+          ],
+          "baselineY": 686,
+          "fontSize": 11,
+          "pageIndex": 0,
+          "text": "dog near the river bank on a sunny",
+          "x0": 72
+        },
+        {
+          "adv": [
+            6.115997314453125,
+            3.0579986572265625,
+            3.0580062866210938,
+            6.115997314453125,
+            3.6630020141601562,
+            6.115997314453125,
+            6.115997314453125,
+            6.116004943847656,
+            6.116004943847656,
+            3.0579986572265625,
+            2.4420013427734375,
+            6.115997314453125,
+            3.0579986572265625,
+            6.115997314453125,
+            6.115997314453125,
+            3.6630096435546875,
+            2.441986083984375,
+            5.5,
+            3.058013916015625,
+            5.5,
+            6.115997314453125,
+            3.662994384765625,
+            2.4420013427734375,
+            6.115997314453125,
+            6.115997314453125,
+            1.100006103515625
+          ],
+          "baselineY": 672,
+          "fontSize": 11,
+          "pageIndex": 0,
+          "text": "afternoon in early spring.",
+          "x0": 72
+        }
+      ]
+    }
+  },
+  "editOneWasm": {
+    "label": "AFTER_ONE_CHAR_WASM",
+    "renderBytes": 33262,
+    "layout": {
+      "anchorPage": 0,
+      "columnLeft": 72.25299835205078,
+      "lines": [
+        {
+          "adv": [
+            5.3604375,
+            5.779296875,
+            5.4731875,
+            2.486859375,
+            5.779296875,
+            5.779296875,
+            2.5244999999999997,
+            4.651453125,
+            5.00053125,
+            2.486859375,
+            5.779296875,
+            3.6578437499999996,
+            5.7578125,
+            7.863281249999999,
+            5.779296875,
+            2.486859375,
+            3.125890625,
+            5.5859375,
+            4.764203125,
+            2.486859375,
+            2.6319218749999997,
+            5.779296875,
+            8.787109375,
+            5.725671875,
+            4.302203125,
+            2.486859375,
+            5.752484375,
+            4.860796875,
+            5.4731875,
+            3.8350468749999997,
+            2.486859375,
+            3.6846562499999997,
+            5.779296875,
+            5.4731875,
+            2.486859375,
+            2.5244999999999997,
+            5.269,
+            4.248578125,
+            4.979046875,
+            4.764203125,
+            2.486859375,
+            5.779296875,
+            5.80078125,
+            5.177734375,
+            2.486859375,
+            5.779296875,
+            5.4731875,
+            5.269,
+            3.8350468749999997,
+            2.486859375,
+            3.6846562499999997,
+            5.779296875,
+            5.4731875
+          ],
+          "baselineY": 700,
+          "fontSize": 11,
+          "pageIndex": 0,
+          "text": "The quick brown fox jumps over the lazyx dog near the",
+          "x0": 72
+        },
+        {
+          "adv": [
+            3.8350468749999997,
+            2.5244999999999997,
+            4.860796875,
+            5.4731875,
+            3.8350468749999997,
+            2.486859375,
+            5.779296875,
+            5.269,
+            5.779296875,
+            5.00053125,
+            2.486859375,
+            5.80078125,
+            5.779296875,
+            2.486859375,
+            5.269,
+            2.486859375,
+            4.302203125,
+            5.779296875,
+            5.779296875,
+            5.57528125,
+            4.979046875,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705,
+            4.2531359953703705
+          ],
+          "baselineY": 686,
+          "fontSize": 11,
+          "pageIndex": 0,
+          "text": "river bank on a sunny afternoon in early spring.",
+          "x0": 72
+        }
+      ]
+    }
+  }
+}
diff --git a/tests/edits/forensic_wasm_out.json b/tests/edits/forensic_wasm_out.json
new file mode 100644
index 0000000..507d776
--- /dev/null
+++ b/tests/edits/forensic_wasm_out.json
@@ -0,0 +1,71 @@
+{
+  "editEntryWasm": {
+    "label": "EDIT_MODE_ENTRY_WASM",
+    "renderBytes": 10936,
+    "layout": {
+      "anchorPage": 0,
+      "columnLeft": 20.847999572753906,
+      "lines": [
+        {
+          "adv": [
+            12.447998046875,
+            8.896003723144531,
+            8.89599609375,
+            8.896003723144531,
+            8.89599609375,
+            8,
+            8.896003723144531,
+            4.447998046875,
+            4.447998046875,
+            11.552001953125,
+            8.896003723144531,
+            5.3280029296875,
+            3.552001953125,
+            8.89599609375,
+            1.743988037109375
+          ],
+          "baselineY": 100,
+          "fontSize": 16,
+          "pageIndex": 0,
+          "text": "Goodbye, world!",
+          "x0": 20
+        }
+      ]
+    }
+  },
+  "editOneWasm": {
+    "label": "AFTER_ONE_CHAR_WASM",
+    "renderBytes": 11454,
+    "layout": {
+      "anchorPage": 0,
+      "columnLeft": 20.847999572753906,
+      "lines": [
+        {
+          "adv": [
+            10.09375,
+            8.4375,
+            8.4375,
+            8.40625,
+            8.32825,
+            7.047,
+            7.961,
+            3.9922500000000003,
+            3.61725,
+            11.289,
+            8.4375,
+            5.57825,
+            3.672,
+            8.40625,
+            5.211,
+            6.92975
+          ],
+          "baselineY": 100,
+          "fontSize": 16,
+          "pageIndex": 0,
+          "text": "Goodbye, world!x",
+          "x0": 20
+        }
+      ]
+    }
+  }
+}
\ No newline at end of file
diff --git a/wasm/CMakeLists.txt b/wasm/CMakeLists.txt
index 57056bf..f0574b6 100644
--- a/wasm/CMakeLists.txt
+++ b/wasm/CMakeLists.txt
@@ -59,7 +59,7 @@ target_link_options(pdfengine_wasm PRIVATE
     "-sALLOW_MEMORY_GROWTH=1"
     "-sWASM_BIGINT"                 
     "-sSTACK_SIZE=5MB"              
-    "-sEXPORTED_FUNCTIONS=['_loadDocument','_pageCount','_renderPagePng','_previewRender','_previewRenderRegion','_previewRenderPaginated','_lastRegionCount','_lastRegionPtr','_lastRegionW','_lastRegionH','_lastRegionPage','_lastRegionYTop','_lastRenderPtr','_lastRenderW','_lastRenderH','_lastLayoutJson','_registerAuxFont','_freeDocument','_engineBuildInfo','_malloc','_free']"
+    "-sEXPORTED_FUNCTIONS=['_loadDocument','_pageCount','_renderPagePng','_previewRender','_previewRenderRegion','_previewRenderPaginated','_lastRegionCount','_lastRegionPtr','_lastRegionW','_lastRegionH','_lastRegionPage','_lastRegionYTop','_lastRenderPtr','_lastRenderW','_lastRenderH','_lastLayoutJson','_registerAuxFont','_freeDocument','_engineBuildInfo','_debugValidateLayout','_malloc','_free']"
     "-sEXPORTED_RUNTIME_METHODS=['ccall','cwrap','getValue','setValue','HEAPU8']"
 )
 
diff --git a/wasm/bindings/wasm_engine.cpp b/wasm/bindings/wasm_engine.cpp
index 51f75b4..a4b3570 100644
--- a/wasm/bindings/wasm_engine.cpp
+++ b/wasm/bindings/wasm_engine.cpp
@@ -164,4 +164,13 @@ EMSCRIPTEN_KEEPALIVE void freeDocument(int handle) { g_docs.erase(handle); }
 
 EMSCRIPTEN_KEEPALIVE const char* engineBuildInfo() { return "pdfengine-wasm+pdfium"; }
 
+std::string g_lastValidation;
+
+EMSCRIPTEN_KEEPALIVE const char* debugValidateLayout(int handle, int pageIndex, const char* jsonStr) {
+    auto it = g_docs.find(handle);
+    if (it == g_docs.end() || !jsonStr) return "{}";
+    g_lastValidation = it->second.doc->validateLayout(pageIndex, jsonStr);
+    return g_lastValidation.c_str();
+}
+
 }
\ No newline at end of file