feat: implemented hit testing, form field, content stream and base security hardening

.github/workflows/fuzz.yml

@@ -0,0 +1,104 @@
+# Extended robustness: corpus render-stability sweep + libFuzzer run.
+#
+# DELIBERATELY ISOLATED from the main CI gate:
+#   - never triggers on push or pull_request, so it can NEVER block a merge,
+#     push, or pull;
+#   - the whole job is continue-on-error, so a crash finding or a build/runner
+#     problem reports red here but does not fail any required check;
+#   - runs on a weekly schedule and on manual dispatch only.
+#
+# A full 24h fuzz run needs a self-hosted runner (GitHub-hosted runners cap a job
+# at 6h). Use the workflow_dispatch `duration_seconds` input for that; the weekly
+# schedule does a short smoke instead.
+
+name: Fuzz & corpus sweep
+
+on:
+  schedule:
+    - cron: "0 3 * * 0" # Sundays 03:00 UTC — short smoke
+  workflow_dispatch:
+    inputs:
+      duration_seconds:
+        description: "libFuzzer -max_total_time (e.g. 1800 smoke, 86400 for 24h on a self-hosted runner)"
+        default: "1800"
+      sanitizers:
+        description: "Sanitizer set (fuzzer,address,undefined needs an ASan/UBSan-built PDFium; fuzzer = coverage-only)"
+        default: "fuzzer,address,undefined"
+
+permissions:
+  contents: read
+
+concurrency:
+  group: fuzz-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  fuzz:
+    # Non-blocking by construction: nothing depends on this job and it is allowed to fail.
+    continue-on-error: true
+    runs-on: ubuntu-latest
+    timeout-minutes: 1500 # permits a 24h dispatch on a self-hosted runner; hosted runners stop at 6h
+    env:
+      VCPKG_DEFAULT_BINARY_CACHE: ${{ github.workspace }}/.vcpkg-cache
+      FUZZ_SANITIZERS: ${{ github.event.inputs.sanitizers || 'fuzzer' }}
+      FUZZ_DURATION: ${{ github.event.inputs.duration_seconds || '600' }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Ninja + Clang
+        shell: bash
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y ninja-build clang
+
+      - name: Locate vcpkg
+        shell: bash
+        run: |
+          echo "VCPKG_ROOT=$VCPKG_INSTALLATION_ROOT" >> "$GITHUB_ENV"
+          mkdir -p "$VCPKG_DEFAULT_BINARY_CACHE"
+
+      - name: Cache vcpkg artifacts
+        uses: actions/cache@v4
+        with:
+          path: ${{ env.VCPKG_DEFAULT_BINARY_CACHE }}
+          key: vcpkg-fuzz-${{ hashFiles('vcpkg.json') }}
+          restore-keys: vcpkg-fuzz-
+
+      - name: Pin vcpkg dependency baseline
+        shell: bash
+        run: |
+          if ! grep -q '"builtin-baseline"' vcpkg.json; then
+            "$VCPKG_ROOT/vcpkg" x-update-baseline --add-initial-baseline
+          fi
+
+      - name: Configure (fuzz-linux)
+        run: cmake --preset fuzz-linux -DPDFENGINE_FUZZ_SANITIZERS="$FUZZ_SANITIZERS"
+
+      - name: Build fuzzer
+        run: cmake --build --preset fuzz-linux --target pdfengine_fuzz
+
+      - name: Fetch corpus (pinned + hash-verified)
+        shell: bash
+        run: python3 scripts/fetch_corpus.py --manifest tests/regression/corpus-manifest.json || echo "corpus fetch failed (network) — continuing with committed corpus"
+
+      - name: Render-stability sweep (replay every corpus PDF once)
+        shell: bash
+        run: |
+          BIN=out/build/fuzz-linux/bin/pdfengine_fuzz
+          mkdir -p engine/fuzz/artifacts
+          "$BIN" -runs=0 -artifact_prefix=engine/fuzz/artifacts/ corpus/ corpus/fuzz/ || true
+
+      - name: Fuzz run
+        shell: bash
+        run: |
+          BIN=out/build/fuzz-linux/bin/pdfengine_fuzz
+          "$BIN" -max_total_time="$FUZZ_DURATION" -print_final_stats=1 \
+                 -artifact_prefix=engine/fuzz/artifacts/ corpus/fuzz/ corpus/ || true
+
+      - name: Upload crash artifacts
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: fuzz-artifacts
+          path: engine/fuzz/artifacts/
+          if-no-files-found: ignore

.gitignore

@@ -67,5 +67,15 @@ gateway/*.pyd
 gateway/*.so
 gateway/*.dylib
 
+# Downloaded fuzzing corpus (large; fetched via scripts/fetch_corpus.py)
+corpus/fuzz/
+# Large-corpus regression baseline (derived from the gitignored corpus above)
+tests/regression/baseline-large/
+# Fuzzer working artifacts (crashes, leaks, coverage)
+engine/fuzz/artifacts/
+crash-*
+leak-*
+timeout-*
+
 PDF Editor Timeline.xlsx
 # Local environment config / secrets

CMakeLists.txt

@@ -40,6 +40,16 @@ option(PDFENGINE_ENABLE_SANITIZERS "Build with AddressSanitizer/UBSan"
 option(PDFENGINE_WARNINGS_AS_ERRORS "Treat compiler warnings as errors"          OFF)
 option(PDFENGINE_WITH_PDFIUM      "Link the PDFium static lib (build it first)"  OFF)
 option(PDFENGINE_WITH_SKIA        "Link the Skia static lib (build it first)"    OFF)
+option(PDFENGINE_FUZZING          "Build libFuzzer harnesses (requires Clang)"   OFF)
+
+if(PDFENGINE_FUZZING)
+    # The fuzz harness instruments the engine; tests/bindings are not part of it.
+    set(PDFENGINE_BUILD_TESTS OFF CACHE BOOL "Build engine unit/smoke tests" FORCE)
+    if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+        message(FATAL_ERROR "PDFENGINE_FUZZING requires a Clang/clang-cl toolchain "
+                            "(set in the 'fuzz-*' preset).")
+    endif()
+endif()
 
 if(PDFENGINE_WASM)
     set(PDFENGINE_BUILD_TESTS OFF CACHE BOOL "Build engine unit/smoke tests" FORCE)
@@ -80,7 +90,9 @@ else()
     endif()
 
     add_subdirectory(engine)
+    if(NOT PDFENGINE_FUZZING)
         add_subdirectory(bindings)
+    endif()
 endif()
 
 message(STATUS "PdfEngine ${PROJECT_VERSION} configured")

CMakePresets.json

@@ -108,6 +108,27 @@
         "PDFENGINE_BUILD_TESTS": "OFF",
         "PDFENGINE_WITH_PDFIUM": "OFF"
       }
+    },
+
+    {
+      "name": "fuzz-linux",
+      "displayName": "Linux • libFuzzer (Clang + ASan)",
+      "inherits": "base",
+      "condition": { "type": "equals", "lhs": "${hostSystemName}", "rhs": "Linux" },
+      "cacheVariables": {
+        "CMAKE_BUILD_TYPE": "RelWithDebInfo",
+        "CMAKE_C_COMPILER": "clang",
+        "CMAKE_CXX_COMPILER": "clang++",
+        "PDFENGINE_FUZZING": "ON",
+        "PDFENGINE_WITH_PDFIUM": "ON",
+        "PDFENGINE_FUZZ_SANITIZERS": "fuzzer,address,undefined"
+      }
+    },
+    {
+      "name": "fuzz-linux-nosan",
+      "displayName": "Linux • libFuzzer (Clang, coverage-only — for non-ASan PDFium)",
+      "inherits": "fuzz-linux",
+      "cacheVariables": { "PDFENGINE_FUZZ_SANITIZERS": "fuzzer" }
     }
   ],
 
@@ -121,7 +142,9 @@
     { "name": "macos-debug",     "configurePreset": "macos-debug" },
     { "name": "macos-release",   "configurePreset": "macos-release" },
     { "name": "macos-asan",      "configurePreset": "macos-asan" },
-    { "name": "wasm",            "configurePreset": "wasm" }
+    { "name": "wasm",            "configurePreset": "wasm" },
+    { "name": "fuzz-linux",       "configurePreset": "fuzz-linux" },
+    { "name": "fuzz-linux-nosan", "configurePreset": "fuzz-linux-nosan" }
   ],
 
   "testPresets": [

bindings/python/pdfengine_py.cpp

@@ -212,6 +212,40 @@ PYBIND11_MODULE(pdfengine, m) {
             }
             return py_list;
         })
+        .def("ordered_glyphs", [](const pdfengine::PdfPage& self) {
+            auto res = get_or_throw(self.orderedGlyphs());
+            py::list py_list;
+            for (const auto& g : res) {
+                py::dict d;
+                d["text"] = g.text; d["x"] = g.x; d["y"] = g.y;
+                d["w"] = g.w; d["h"] = g.h; d["fontSize"] = g.fontSize;
+                py_list.append(d);
+            }
+            return py_list;
+        })
+        .def("hit_glyph", [](const pdfengine::PdfPage& self, double x, double y) {
+            auto hit = get_or_throw(self.hitGlyph(x, y));
+            py::dict d;
+            d["glyphIndex"] = hit.glyphIndex;
+            d["caret"] = hit.caret;
+            d["line"] = hit.line;
+            return d;
+        }, py::arg("x"), py::arg("y"))
+        .def("select_range", [](const pdfengine::PdfPage& self, double ax, double ay, double bx, double by) {
+            auto sel = get_or_throw(self.selectRange(ax, ay, bx, by));
+            py::dict d;
+            d["startGlyph"] = sel.startGlyph;
+            d["endGlyph"] = sel.endGlyph;
+            d["text"] = sel.text;
+            py::list rects;
+            for (const auto& r : sel.rects) {
+                py::dict rd;
+                rd["x"] = r.x; rd["y"] = r.y; rd["w"] = r.w; rd["h"] = r.h;
+                rects.append(rd);
+            }
+            d["rects"] = rects;
+            return d;
+        }, py::arg("ax"), py::arg("ay"), py::arg("bx"), py::arg("by"))
         .def("get_fonts", [](const pdfengine::PdfPage& self) {
             return get_or_throw(self.getFonts());
         })

docs/phase3-dev1-plan.md

@@ -0,0 +1,130 @@
+# Phase 3 — Dev-1 Plan (Form Filling · Hit-Testing · Test Harness) — v2
+
+**Status:** Review before build (reworked with your decisions). No code changed. Grounded in the current codebase (verified).
+**Scope:** the three Phase-3 items not blocked by the content-stream analyser. Build them in parallel with (separate) content-stream work.
+
+## Locked decisions (from review)
+1. **Form fill UI → Acrobat-style on-page overlay inputs** (edit fields directly on the page), built now — not the lighter in-panel version.
+2. **Form rendering → production-grade** (`FPDF_FFLDraw` + proper appearance regeneration; robust per-type).
+3. **Hit-testing → build BOTH** the engine `hitGlyph`/spatial index (the roadmap Dev-1 deliverable — *not* deferred, since it's not scheduled in any other phase) **and** the frontend real-time selection model. They share one algorithm; the engine is authoritative/tested/SDK-exposed, the frontend is the responsive web layer.
+4. **Regression gate → freeze PDFium baseline now**, becomes meaningful when the custom renderer lands.
+5. **Fuzzing → Windows AND Linux** (clang-cl on Windows + clang on Linux).
+6. **Corpus → download script** into a gitignored dir (500+ public PDFs).
+
+> None of these need the content-stream analyser. They build on Phase-1 glyphs, the form-field reader, and the existing gtest/ASan infra.
+
+---
+
+## Workstream A — Production-Grade Form Filling + Acrobat-Style On-Page Editing
+
+### Current state (verified)
+- ✅ `update_field` handler exists ([pdfium_document.cpp:1809](engine/src/parser/pdfium_document.cpp#L1809)) — sets `/V`, bool→checkbox, saves. **~50%.**
+- ✅ Form-fill env initialized; field reader returns name/value/type/flags/options; Forms tab lists them (read-only).
+- ❌ `render()` uses `flags=0` ([line 717](engine/src/parser/pdfium_document.cpp#L717)) → form widgets not drawn (filled values invisible).
+- ❌ No `/AS` for checkboxes, no `FORM_SetIndexSelected` for choices, no appearance regen, no on-page edit UI.
+
+### Build — Engine (C++)
+1. **Render forms (production):** retain the form-fill env on the document; in `render()` call **`FPDF_FFLDraw(formHandle, bitmap, page, x, y, w, h, rotate, FPDF_ANNOT)`** after `FPDF_RenderPageBitmap` so field appearances (incl. filled values) composite into the page image.
+2. **Complete `update_field` per field type:**
+   - **Text (Tx):** set `/V`; regenerate appearance (via form env `FORM_ReplaceSelection`/`FORM_ForceToKillFocus`, or set AcroForm `/NeedAppearances`) so the value renders + exports.
+   - **Checkbox/Radio (Btn):** set `/V` **and** `/AS` to the field's on-state name (read the AP states); radios update the sibling group.
+   - **Choice (Ch):** combo → `/V`; listbox/multi → `FORM_SetIndexSelected(formHandle, page, index, selected)` (+ `/V`).
+3. **Robust field identity:** match by the same id convention as `delete_annotation` (`NM` else `anno_<page>_<idx>`), not raw index parsing.
+4. **Save:** incremental (form edits are dict changes); `save_full` on export.
+
+### Build — Gateway
+- Extend `UpdateFieldData` if needed: `value: str | bool`, optional `selectedIndices: int[]` (listbox multi-select).
+- (Optional) a `GET /…/form-fields` convenience endpoint — or keep reading them from `/annotations` (already includes field props).
+
+### Build — Frontend (Acrobat-style on-page editing)
+1. **`FormFieldLayer`** (new viewer layer, sibling to AnnotationLayer): for each `widget` annotation on the page, render an **HTML control positioned over the field rect** (×zoom), type-mapped:
+   - Tx → `<input>` / `<textarea>` (multiline per `fieldFlags`)
+   - Btn checkbox → `<input type=checkbox>`; radio → grouped radios
+   - Ch combo → `<select>` (or input+datalist if editable); listbox → `<select multiple>`
+   - Sig → click-to-sign placeholder (reuses Signature flow)
+2. **Positioning:** field rect comes from `extractAnnotations` (top-left device space) → place + scale with zoom, like the other layers; transparent styling so the PDF field box shows through.
+3. **Interaction:** edit on the page → on blur/change emit `update_field` → optimistic value + save → page re-renders with baked appearance (so it's consistent on export).
+4. **Modes:** a "Fill & Sign" affordance to toggle the form layer on (so inputs don't interfere with other tools).
+
+### Acceptance
+On a form PDF you can **type into text fields, tick checkboxes, pick dropdowns directly on the page**; values persist after save, **render in the page image**, and survive export. Verified on `text_form` / `combobox_form` / `listbox_form`.
+
+### Risk / effort
+PDFium form rendering + appearance regen is the classic gotcha (`/AS` state names vary; multiline/comb fields). On-page overlay positioning across zoom/rotation needs care. **~1.5–2 weeks.**
+
+---
+
+## Workstream B — Adobe-Level Hit-Testing & Text Selection (engine + frontend)
+
+### Current state (verified)
+- ✅ Glyph bounds (`extractTextWithBounds`).
+- 🟡 `SelectionLayer` = rectangular marquee (`rectsIntersect`) — not reading-order selection.
+
+### Build — Engine (the roadmap Dev-1 deliverable; authoritative + SDK)
+1. **Reading-order model per page:** line detection (cluster by baseline/y), order glyphs L→R within line, top→bottom across lines; assign sequential index. (v1 = LTR single/simple multi-column; note RTL/complex as limitation.)
+2. **Spatial index:** sorted-vector + binary search (or uniform grid) over glyph rects.
+3. **APIs (bound to Python/SDK):**
+   - `hit_glyph(x, y) → {glyphIndex, caretSide}` (nearest glyph + before/after).
+   - `select_range(p1, p2) → {glyphIndices[], text}` (ordered range between two points).
+   - `word_at(x,y)`, `line_at(x,y)`.
+4. **Tests:** corpus-based accuracy (hit/range correctness; reading order sanity).
+
+### Build — Frontend (Adobe-grade interactive selection)
+1. **`TextSelectionModel`** (client-side): mirrors the engine's reading-order + spatial index for **real-time** interaction (no HTTP per mouse-move). Shares the algorithm; an automated test compares frontend vs engine selection on the corpus to prevent drift.
+2. **Interactions (Adobe parity):**
+   - Drag = **anchor→focus reading-order range** (flows across lines, partial first/last line).
+   - **Double-click = word**, **triple-click = line/paragraph**.
+   - **Shift-click / Shift-drag = extend**; caret position; **Ctrl/⌘+A = select page**.
+   - **Copy** = ordered text with correct spaces/newlines.
+   - *(Stretch)* cross-page selection (Adobe does this; v1 may be per-page — flagged).
+3. **Visual:** selection rendered as highlighted glyph runs (smooth at any zoom).
+
+### Acceptance
+Selection behaves like Acrobat/a browser: drag selects a flowing range across lines, double/triple-click select word/line, shift extends, copy yields correctly-ordered text; smooth on a dense/100-page doc. Engine `hit_glyph`/`select_range` exposed + tested. **Also the foundation for "click a glyph to edit it" once content-stream editing lands.**
+
+### Risk / effort
+Reading order for columns/RTL is hard — v1 targets LTR; complex layouts flagged. Engine + frontend + parity test. **~1.5 weeks.**
+
+---
+
+## Workstream C — Fuzz + Regression Harness (Windows + Linux)
+
+### Current state (verified)
+- ✅ 41 corpus PDFs; gtest suite (`engine/tests/`); CTest; ASan/UBSan presets (`windows-asan`/`linux-asan`).
+
+### Build — Regression suite
+1. `tests/regression/` — Python harness: render each corpus PDF (engine bindings) at 144 DPI → PNG → **SSIM vs `baseline/`**; fail < 0.95; HTML/JSON report.
+2. **Freeze the PDFium baseline now** (golden images). When the **custom renderer** lands, switch engine-render to the custom path → gate becomes the real safety net.
+3. `scripts/fetch_corpus.ps1`/`.sh` — pull 500+ permissive public PDFs (pdf.js / pdfium test resources) into a **gitignored** corpus dir.
+4. CI: regression job wired as a gate.
+
+### Build — Fuzzing (Windows + Linux)
+1. `tests/fuzz/fuzz_load_render.cpp` — `LLVMFuzzerTestOneInput(data,size)` → `loadFromMemory` → page/render/extractText/applyEdits; seed = the 41 PDFs.
+2. **Toolchain:** **Linux** = clang `-fsanitize=fuzzer,address,undefined`; **Windows** = **clang-cl** `-fsanitize=fuzzer,address` (UBSan partial on Windows — run full UBSan on Linux). New CMake `fuzz` preset(s) (`fuzz-linux`, `fuzz-windows`).
+3. `engine/include/pdfengine/hardened_limits.h` — `MAX_OBJECTS` / `MAX_STREAM_SIZE` / `MAX_PAGE_COUNT` checked in load/parse paths.
+4. Runs: nightly fuzz (both OSes), ASan/UBSan over the existing gtest suite via the `*-asan` presets.
+
+### Acceptance
+CI runs SSIM regression (green baseline now); fuzz targets **build and run on Windows + Linux**, N hours zero-crash, ASan clean; UBSan clean on Linux. `hardened_limits` guards enforced.
+
+### Risk / effort
+clang-cl + libFuzzer on Windows needs LLVM installed (the MSVC `windows-asan` preset stays for unit-test ASan). Corpus licensing → permissive sets only. **~1–1.5 weeks, mostly background/parallel.**
+
+---
+
+## Build order & parallelization
+1. **A — Form filling** first (real Adobe feature; one engine rebuild; production-grade on-page editing).
+2. **B — Hit-testing/selection** next (engine spatial index + frontend Adobe selection; sets up future text editing).
+3. **C — Harness** stood up from day 1 in the background (protects later Dev-2/Dev-3 content-stream work).
+
+All independent of each other and of the content-stream analyser. Each ends with: engine rebuild (A, B-engine) + frontend typecheck/build + end-to-end verification, and the gateway test instance stopped afterward.
+
+## Revised effort
+| Workstream | Effort | Engine rebuild? |
+|---|---|---|
+| A — Form filling (on-page, production) | ~1.5–2 wk | yes |
+| B — Hit-testing (engine + frontend Adobe selection) | ~1.5 wk | yes |
+| C — Fuzz + regression (Win + Linux) | ~1–1.5 wk | new fuzz preset |
+
+## What this delivers
+Form **filling** (Acrobat-style, on-page), **Adobe-grade text selection/copy** (engine-authoritative + responsive UI), and a **production quality safety net** (regression + fuzz on Windows & Linux). Still **not** editing existing page text/images — that's the content-stream chain (separate critical path).

engine/CMakeLists.txt

@@ -17,6 +17,7 @@ add_library(pdfengine STATIC
     src/parser/pdfium_loader.cpp
     src/parser/pdfium_document.cpp
     src/parser/content_stream_parser.cpp
+    src/text/selection.cpp
     src/fonts/face/font_face.cpp
     src/fonts/face/free_type_manager.cpp
     src/fonts/loader/font_resolver.cpp
@@ -72,3 +73,39 @@ pdfengine_enable_sanitizers(pdfengine)
 if(PDFENGINE_BUILD_TESTS)
     add_subdirectory(tests)
 endif()
+
+# --- Fuzzing -----------------------------------------------------------------
+# Coverage-instrument the engine and build the libFuzzer harness. Requires Clang
+# (enforced at the top level). PDFENGINE_FUZZ_SANITIZERS lets callers drop ASan
+# when the PDFium static lib isn't ASan-compatible (coverage-only fuzzing).
+if(PDFENGINE_FUZZING)
+    set(PDFENGINE_FUZZ_SANITIZERS "fuzzer,address,undefined" CACHE STRING
+        "Sanitizer set for fuzzing (e.g. 'fuzzer,address,undefined' or just 'fuzzer')")
+
+    # Split the set into the libFuzzer driver ('fuzzer', linked only into the
+    # harness exe) and the runtime sanitizers (address/undefined/...), which must
+    # instrument the engine library itself to catch bugs in its code.
+    string(REPLACE "," ";" _fuzz_sans "${PDFENGINE_FUZZ_SANITIZERS}")
+    set(_fuzz_runtime_sans "")
+    foreach(_s IN LISTS _fuzz_sans)
+        if(NOT _s STREQUAL "fuzzer")
+            list(APPEND _fuzz_runtime_sans "${_s}")
+        endif()
+    endforeach()
+    list(JOIN _fuzz_runtime_sans "," _fuzz_runtime_str)
+
+    # Coverage-instrument the engine; apply ASan/UBSan to it too so its own code
+    # is checked, not just the harness.
+    target_compile_options(pdfengine PRIVATE -fsanitize=fuzzer-no-link -fno-omit-frame-pointer)
+    if(_fuzz_runtime_str)
+        target_compile_options(pdfengine PRIVATE -fsanitize=${_fuzz_runtime_str})
+        target_link_options(pdfengine PUBLIC -fsanitize=${_fuzz_runtime_str})
+    endif()
+
+    add_executable(pdfengine_fuzz fuzz/fuzz_load.cpp)
+    target_link_libraries(pdfengine_fuzz PRIVATE pdfengine)
+    target_compile_options(pdfengine_fuzz PRIVATE
+        -fsanitize=${PDFENGINE_FUZZ_SANITIZERS} -fno-omit-frame-pointer)
+    target_link_options(pdfengine_fuzz PRIVATE
+        -fsanitize=${PDFENGINE_FUZZ_SANITIZERS})
+endif()

engine/fuzz/README.md

@@ -0,0 +1,65 @@
+# Fuzzing the PDF engine
+
+`fuzz_load.cpp` is a libFuzzer harness that drives the full
+**load → metadata → outline → render → text → annotations → hit-test → select**
+path with arbitrary bytes. Combined with AddressSanitizer it surfaces crashes,
+OOMs, and undefined behaviour in the parsing and rendering code.
+
+Resource ceilings from `pdfengine/hardened_limits.h` keep the fuzzer focused on
+logic bugs instead of trivial out-of-memory inputs (and those same ceilings now
+guard the production render path against integer-overflow / OOM).
+
+## Linux (primary)
+
+Clang + libFuzzer + ASan is best supported on Linux. PDFium must be built with
+the same Clang toolchain (so ASan is consistent across the static lib).
+
+```bash
+# Full ASan + coverage fuzzer
+cmake --preset fuzz-linux
+cmake --build --preset fuzz-linux
+
+# If your PDFium static lib is NOT ASan-instrumented, use coverage-only:
+cmake --preset fuzz-linux-nosan
+cmake --build --preset fuzz-linux-nosan
+
+# Run it against the downloaded corpus as a seed set
+python scripts/fetch_corpus.py                  # populates corpus/fuzz/ (gitignored)
+mkdir -p engine/fuzz/artifacts
+./out/build/fuzz-linux/bin/pdfengine_fuzz \
+    -artifact_prefix=engine/fuzz/artifacts/ \
+    corpus/fuzz/ corpus/
+```
+
+`corpus/fuzz/` and `corpus/` are passed as seed corpora; new coverage-expanding
+inputs are written back into the first directory. Crashes land in
+`engine/fuzz/artifacts/` (gitignored).
+
+## Windows (clang-cl)
+
+Native Windows fuzzing needs a Clang toolchain *and* a PDFium static lib built
+with the matching runtime. Configure with clang-cl and the existing
+`x64-windows-static` triplet, then enable fuzzing:
+
+```powershell
+cmake -S . -B C:/Users/<you>/pdfeng-build/fuzz-win -G Ninja `
+  -DCMAKE_C_COMPILER=clang-cl -DCMAKE_CXX_COMPILER=clang-cl `
+  -DVCPKG_TARGET_TRIPLET=x64-windows-static `
+  -DPDFENGINE_FUZZING=ON -DPDFENGINE_WITH_PDFIUM=ON `
+  -DPDFENGINE_FUZZ_SANITIZERS=fuzzer `
+  --toolchain "$env:VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake"
+cmake --build C:/Users/<you>/pdfeng-build/fuzz-win
+```
+
+Use `PDFENGINE_FUZZ_SANITIZERS=fuzzer` (coverage-only) on Windows unless the
+whole dependency chain — including PDFium — is ASan-built, since mixing an
+ASan binary with a non-ASan MSVC static lib does not link cleanly.
+
+## Reproducing a crash
+
+```bash
+./pdfengine_fuzz engine/fuzz/artifacts/crash-<hash>
+```
+
+The ASan report points at the offending allocation/access; the input file is the
+minimal reproducer (run with `-minimize_crash=1` to shrink further).

engine/fuzz/fuzz_load.cpp

@@ -0,0 +1,50 @@
+// libFuzzer entry point: drive the full load → inspect → render → select path
+// with arbitrary bytes, so the fuzzer can find crashes, OOMs, and UB in the
+// PDF parsing and rendering code.
+//
+// Build with a Clang toolchain via the `fuzz-linux` preset (see engine/fuzz/
+// README.md). Every operation is wrapped so a clean error never aborts the run —
+// only a real crash (caught by the sanitizer) should stop the fuzzer.
+
+#include "pdfengine/hardened_limits.h"
+#include "pdfengine/pdf_document.hpp"
+
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+    using namespace pdfengine;
+
+    if (!limits::documentSizeOk(size)) return 0;
+
+    std::vector<uint8_t> bytes(data, data + size);
+    auto doc = PdfDocument::loadFromMemory(bytes, "");
+    if (!doc) return 0;
+    PdfDocument& d = **doc;
+
+    const int pages = d.pageCount();
+    if (!limits::pageCountOk(pages)) return 0;
+
+    (void)d.metadata();
+    (void)d.extractOutline();
+
+    const int limit = pages < 3 ? pages : 3; // bound work per input
+    for (int i = 0; i < limit; ++i) {
+        auto page = d.getPage(i);
+        if (!page) continue;
+        PdfPage& p = **page;
+
+        (void)p.render(72);
+        (void)p.extractText();
+        (void)p.extractAnnotations();
+
+        auto glyphs = p.orderedGlyphs();
+        if (glyphs && !glyphs->empty()) {
+            const auto& g = glyphs->front();
+            (void)p.hitGlyph(g.x, g.y);
+            (void)p.selectRange(g.x, g.y, g.x + 50.0, g.y + 20.0);
+        }
+    }
+    return 0;
+}

engine/include/pdfengine/hardened_limits.h

@@ -0,0 +1,67 @@
+// Resource limits for hardening against malicious / malformed PDFs.
+//
+// These guard the allocation-sizing arithmetic in the load and render paths
+// against integer overflow and pathological out-of-memory inputs (a 2-billion-pt
+// page, a million-page document, a multi-gigabyte raster). The ceilings are set
+// far above anything a legitimate document needs, so enforcing them never
+// rejects real files — they exist purely to turn "crash / OOM" into a clean,
+// recoverable error, which is exactly what a fuzzer needs to make progress.
+//
+// Header-only and dependency-free so the fuzz harness and the engine share one
+// source of truth.
+
+#ifndef PDFENGINE_HARDENED_LIMITS_H
+#define PDFENGINE_HARDENED_LIMITS_H
+
+#include <cstdint>
+
+namespace pdfengine::limits {
+
+// Largest input document we will even attempt to parse (1 GiB).
+inline constexpr std::uint64_t kMaxDocumentBytes = 1ull << 30;
+
+// PDF hard-caps a page at 14,400 user units (200 in) per side; allow a very
+// generous multiple of that to tolerate odd-but-real documents.
+inline constexpr double kMaxPageDimensionPt = 200'000.0; // ~2,777 inches
+
+// No legitimate document has this many pages; stops runaway iteration.
+inline constexpr int kMaxPageCount = 100'000;
+
+// MAX_OBJECTS: ceiling on the number of content objects on a single page. Guards
+// against content-stream "object bombs" that would explode parsing/rendering
+// time and memory. No real page comes near this.
+inline constexpr int kMaxObjects = 5'000'000;
+
+// Cap a single rasterised page at ~256 megapixels (≈1 GiB at 4 bytes/px). At
+// 96 dpi that is roughly a 16k × 16k page — well beyond any real render.
+inline constexpr std::int64_t kMaxRasterPixels = 256ll * 1024 * 1024;
+
+// True if a raw page size (in points) is sane to render.
+inline constexpr bool pageDimensionsOk(double widthPt, double heightPt) noexcept {
+    return widthPt > 0.0 && heightPt > 0.0 && widthPt <= kMaxPageDimensionPt &&
+           heightPt <= kMaxPageDimensionPt;
+}
+
+// True if a target raster (in pixels) fits the pixel budget without overflowing
+// the width*height*4 byte computation.
+inline constexpr bool rasterSizeOk(std::int64_t widthPx, std::int64_t heightPx) noexcept {
+    if (widthPx <= 0 || heightPx <= 0) return false;
+    if (widthPx > kMaxRasterPixels || heightPx > kMaxRasterPixels) return false;
+    return widthPx * heightPx <= kMaxRasterPixels;
+}
+
+inline constexpr bool documentSizeOk(std::uint64_t bytes) noexcept {
+    return bytes > 0 && bytes <= kMaxDocumentBytes;
+}
+
+inline constexpr bool pageCountOk(int pages) noexcept {
+    return pages >= 0 && pages <= kMaxPageCount;
+}
+
+inline constexpr bool objectCountOk(int objects) noexcept {
+    return objects >= 0 && objects <= kMaxObjects;
+}
+
+} // namespace pdfengine::limits
+
+#endif // PDFENGINE_HARDENED_LIMITS_H

engine/include/pdfengine/pdf_document.hpp

@@ -53,6 +53,23 @@ struct GlyphBounds {
     double fontSize;
 };
 
+// Result of a point hit-test against a page's glyphs (page-point space, top-left
+// origin — the same space GlyphBounds use).
+struct HitResult {
+    int glyphIndex = -1; // glyph directly under the point in reading order, -1 if none
+    int caret = 0;       // nearest caret position (0..N) for selection anchoring
+    int line = -1;       // line band the point resolved to, -1 if the page has no text
+};
+
+// A resolved text selection: a half-open glyph range plus its reconstructed text
+// and per-line union rectangles (for drawing the selection).
+struct TextSelection {
+    int startGlyph = 0; // inclusive, reading order
+    int endGlyph = 0;   // exclusive
+    std::string text;
+    std::vector<GlyphBounds> rects; // per-line union rects (text field left empty)
+};
+
 struct FontInfo {
     std::string fontName;
     std::string type;              // "TrueType", "Type1", "CIDFontType0", "CIDFontType2"
@@ -136,6 +153,20 @@ public:
 
     [[nodiscard]] virtual std::expected<std::vector<GlyphBounds>, EngineError> extractTextWithBounds() const = 0;
 
+    // Adobe-grade hit-testing & selection, layered on extractTextWithBounds().
+    // Concrete (non-virtual) so every page implementation gets them for free.
+    // Coordinates are in page-point space (top-left origin), matching GlyphBounds.
+
+    // Glyphs in reading order: clustered into lines top-to-bottom, left-to-right.
+    [[nodiscard]] std::expected<std::vector<GlyphBounds>, EngineError> orderedGlyphs() const;
+
+    // Nearest glyph/caret to a point.
+    [[nodiscard]] std::expected<HitResult, EngineError> hitGlyph(double x, double y) const;
+
+    // Reading-order selection between two points (e.g. drag anchor → focus).
+    [[nodiscard]] std::expected<TextSelection, EngineError>
+    selectRange(double ax, double ay, double bx, double by) const;
+
     [[nodiscard]] virtual std::expected<PageModel, EngineError> extractDocumentModel() const = 0;
 
     [[nodiscard]] virtual std::expected<std::vector<FontInfo>, EngineError> getFonts() const = 0;

engine/src/parser/pdfium_document.cpp

@@ -6,12 +6,14 @@
 #include <fpdf_doc.h>
 #include <fpdf_edit.h>
 #include <fpdf_annot.h>
+#include <fpdf_formfill.h>
 #include <png.h>
 #include "parser/pdfium_loader.hpp"
 #endif
 
 #include "fonts/loader/font_resolver.hpp"
 #include "fonts/pdf_fonts/font.hpp"
+#include "pdfengine/hardened_limits.h"
 
 #include <nlohmann/json.hpp>
 #include <spdlog/spdlog.h>
@@ -622,6 +624,13 @@ PdfDocument::loadFromMemory(const std::vector<uint8_t>& data, const std::string&
     if (data.empty()) {
         return std::unexpected(EngineError::InvalidFormat);
     }
+    // Hardened load-path guard (MAX_STREAM_SIZE): refuse absurdly large inputs
+    // before handing them to the parser.
+    if (!limits::documentSizeOk(data.size())) {
+        spdlog::error("Refusing to load PDF: {} bytes exceeds hardened limit ({} bytes)",
+                      data.size(), limits::kMaxDocumentBytes);
+        return std::unexpected(EngineError::InvalidFormat);
+    }
     std::vector<uint8_t> buffer_copy = data;
     FPDF_DOCUMENT doc = FPDF_LoadMemDocument(buffer_copy.data(), static_cast<int>(buffer_copy.size()),
                                              password.empty() ? nullptr : password.c_str());
@@ -630,6 +639,13 @@ PdfDocument::loadFromMemory(const std::vector<uint8_t>& data, const std::string&
         spdlog::error("Failed to load PDF from memory (error code: {})", err);
         return std::unexpected(mapPdfiumError(err, !password.empty()));
     }
+    // Reject documents with an implausible page count (runaway iteration guard).
+    if (!limits::pageCountOk(FPDF_GetPageCount(doc))) {
+        spdlog::error("Refusing to load PDF: page count exceeds hardened limit ({})",
+                      limits::kMaxPageCount);
+        FPDF_CloseDocument(doc);
+        return std::unexpected(EngineError::InvalidFormat);
+    }
     return std::make_shared<parser::PdfiumDocument>(doc, std::move(buffer_copy));
 #else
     (void)data;
@@ -643,8 +659,9 @@ PdfDocument::loadFromMemory(const std::vector<uint8_t>& data, const std::string&
 
 namespace pdfengine::parser {
 
-PdfiumPage::PdfiumPage(NativeDocHandle docHandle, NativePageHandle pageHandle, int pageIndex)
-    : doc_(docHandle), page_(pageHandle), pageIndex_(pageIndex) {
+PdfiumPage::PdfiumPage(NativeDocHandle docHandle, NativePageHandle pageHandle, int pageIndex,
+                       std::shared_ptr<PdfiumDocument> owner)
+    : doc_(docHandle), page_(pageHandle), pageIndex_(pageIndex), ownerDoc_(std::move(owner)) {
 }
 
 PdfiumPage::~PdfiumPage() {
@@ -670,10 +687,13 @@ PdfiumPage& PdfiumPage::operator=(PdfiumPage&& other) noexcept {
         if (textPage_) FPDFText_ClosePage(textPage_);
         if (page_) FPDF_ClosePage(page_);
 #endif
+        doc_ = other.doc_;
         page_ = other.page_;
         textPage_ = other.textPage_;
         pageIndex_ = other.pageIndex_;
+        ownerDoc_ = std::move(other.ownerDoc_);
 
+        other.doc_ = nullptr;
         other.page_ = nullptr;
         other.textPage_ = nullptr;
         other.pageIndex_ = 0;
@@ -703,10 +723,27 @@ std::expected<PageImage, EngineError> PdfiumPage::render(int dpi) const {
         return std::unexpected(EngineError::Unknown);
     }
 
+    // Reject pathological page sizes before sizing the raster, so a malicious
+    // page can't overflow w*h*4 or trigger a multi-gigabyte allocation.
+    if (!limits::pageDimensionsOk(width(), height())) {
+        return std::unexpected(EngineError::RenderFailed);
+    }
+
+    // MAX_OBJECTS guard: refuse to render a content-stream "object bomb".
+    if (!limits::objectCountOk(FPDFPage_CountObjects(page_))) {
+        spdlog::error("Refusing to render page: object count exceeds hardened limit ({})",
+                      limits::kMaxObjects);
+        return std::unexpected(EngineError::RenderFailed);
+    }
+
     double scale = dpi / 72.0;
     int w = static_cast<int>(width() * scale);
     int h = static_cast<int>(height() * scale);
 
+    if (!limits::rasterSizeOk(w, h)) {
+        return std::unexpected(EngineError::RenderFailed);
+    }
+
     FPDF_BITMAP bitmap = FPDFBitmap_Create(w, h, 1);
     if (!bitmap) {
         return std::unexpected(EngineError::RenderFailed);
@@ -716,6 +753,11 @@ std::expected<PageImage, EngineError> PdfiumPage::render(int dpi) const {
 
     FPDF_RenderPageBitmap(bitmap, page_, 0, 0, w, h, 0, 0);
 
+    // Note: form-field widgets are rendered as an interactive HTML overlay in the
+    // frontend (AnnotationLayer), not baked here — keeps them editable and avoids
+    // double-rendering. `update_field` regenerates the field /AP so exported PDFs
+    // (and external viewers) still show filled values.
+
     const auto* buffer = static_cast<const uint8_t*>(FPDFBitmap_GetBuffer(bitmap));
     int stride = FPDFBitmap_GetStride(bitmap);
 
@@ -1589,7 +1631,9 @@ std::expected<std::shared_ptr<PdfPage>, EngineError> PdfiumDocument::getPage(int
         std::lock_guard<std::mutex> lock(pageCacheMutex_);
         auto it = pageCache_.find(pageIndex);
         if (it != pageCache_.end()) {
-            return it->second;
+            if (auto cached = it->second.lock()) {
+                return cached;
+            }
         }
     }
 
@@ -1598,7 +1642,11 @@ std::expected<std::shared_ptr<PdfPage>, EngineError> PdfiumDocument::getPage(int
         return std::unexpected(EngineError::Unknown);
     }
 
-    auto pageObj = std::make_shared<PdfiumPage>(doc_, pageHandle, pageIndex);
+    // The page co-owns this document (shared_from_this) so the native
+    // FPDF_DOCUMENT outlives every page derived from it.
+    auto pageObj = std::make_shared<PdfiumPage>(
+        doc_, pageHandle, pageIndex,
+        std::static_pointer_cast<PdfiumDocument>(shared_from_this()));
     {
         std::lock_guard<std::mutex> lock(pageCacheMutex_);
         pageCache_[pageIndex] = pageObj;
@@ -1805,6 +1853,182 @@ std::expected<void, EngineError> PdfiumDocument::applyEdits(const std::string& e
                     return std::unexpected(EngineError::Unknown);
                 }
 
+                FPDF_ClosePage(page);
+            } else if (type == "edit_text") {
+                // Rewrite an EXISTING text object in place (true content editing).
+                // Locate the target text object by bbox (no stable object id exists),
+                // replace its text, and horizontally squeeze it to stay within the
+                // original line bounds (line-level reflow only).
+                if (!op.contains("data") || !op["data"].is_object()) {
+                    spdlog::error("edit_text operation missing 'data' object");
+                    return std::unexpected(EngineError::InvalidFormat);
+                }
+                auto data = op["data"];
+                double x = data.value("x", 0.0);
+                double y = data.value("y", 0.0);
+                double width = data.value("width", 0.0);
+                double height = data.value("height", 0.0);
+                std::string newText = data.value("newText", "");
+                double fontSize = data.value("fontSize", 0.0);
+                const bool hasColor = data.contains("color") && data["color"].is_string();
+                std::string color = hasColor ? data["color"].get<std::string>() : "#000000";
+                std::string fallbackFont = data.value("fallbackFont", "");
+
+                if (newText.empty()) {
+                    spdlog::warn("edit_text: empty newText — skipping (use redaction to delete text)");
+                    continue;
+                }
+                if (width <= 0.0 || height <= 0.0) {
+                    spdlog::error("edit_text: invalid target bbox ({}x{})", width, height);
+                    return std::unexpected(EngineError::InvalidFormat);
+                }
+
+                FPDF_PAGE page = FPDF_LoadPage(doc_, pageIndex);
+                if (!page) {
+                    spdlog::error("Failed to load page index {} for edit_text", pageIndex);
+                    return std::unexpected(EngineError::Unknown);
+                }
+
+                // --- locate best-matching text object by bbox overlap ---
+                const double tl = x, tb = y, tr = x + width, tt = y + height;
+                const double targetArea = width * height;
+                FPDF_PAGEOBJECT best = nullptr;
+                double bestScore = 0.0, secondScore = 0.0;
+                float bL = 0, bB = 0, bR = 0, bT = 0;
+                const int count = FPDFPage_CountObjects(page);
+                for (int i = 0; i < count; ++i) {
+                    FPDF_PAGEOBJECT obj = FPDFPage_GetObject(page, i);
+                    if (!obj || FPDFPageObj_GetType(obj) != FPDF_PAGEOBJ_TEXT) continue;
+                    float l = 0, b = 0, r = 0, t = 0;
+                    if (!FPDFPageObj_GetBounds(obj, &l, &b, &r, &t)) continue;
+                    const double ix = (std::max)(0.0, (std::min)(static_cast<double>(r), tr) - (std::max)(static_cast<double>(l), tl));
+                    const double iy = (std::max)(0.0, (std::min)(static_cast<double>(t), tt) - (std::max)(static_cast<double>(b), tb));
+                    const double inter = ix * iy;
+                    if (inter <= 0.0) continue;
+                    const double objArea = (std::max)(1e-6, static_cast<double>(r - l) * static_cast<double>(t - b));
+                    const double score = (std::max)(inter / (targetArea + objArea - inter), inter / objArea);
+                    if (score > bestScore) {
+                        secondScore = bestScore;
+                        bestScore = score;
+                        best = obj; bL = l; bB = b; bR = r; bT = t;
+                    } else if (score > secondScore) {
+                        secondScore = score;
+                    }
+                }
+                if (!best || bestScore < 0.30) {
+                    spdlog::error("edit_text: no text object matches the target bbox (best score {:.3f})", bestScore);
+                    FPDF_ClosePage(page);
+                    return std::unexpected(EngineError::InvalidFormat);
+                }
+                if (bestScore - secondScore < 0.10) {
+                    spdlog::error("edit_text: ambiguous target — overlapping text objects (best {:.3f}, second {:.3f})",
+                                  bestScore, secondScore);
+                    FPDF_ClosePage(page);
+                    return std::unexpected(EngineError::InvalidFormat);
+                }
+
+                // --- capture original geometry BEFORE mutating ---
+                const double origLeft = bL, origBottom = bB, origWidth = static_cast<double>(bR - bL);
+                if (fontSize <= 0.0) fontSize = static_cast<double>(bT - bB);
+                if (fontSize <= 0.0) fontSize = 12.0;
+                FS_MATRIX m0{1, 0, 0, 1, 0, 0};
+                FPDFPageObj_GetMatrix(best, &m0);
+                const bool axisAligned = (std::abs(m0.b) < 1e-6 && std::abs(m0.c) < 1e-6);
+
+                // --- glyph-coverage check on the object's own font ---
+                auto decodeUtf8 = [](const std::string& s) {
+                    std::vector<uint32_t> cps;
+                    for (size_t i = 0; i < s.size();) {
+                        unsigned char c = s[i];
+                        uint32_t cp = 0; size_t extra = 0;
+                        if (c < 0x80) { cp = c; extra = 0; }
+                        else if ((c & 0xE0) == 0xC0) { cp = c & 0x1F; extra = 1; }
+                        else if ((c & 0xF0) == 0xE0) { cp = c & 0x0F; extra = 2; }
+                        else if ((c & 0xF8) == 0xF0) { cp = c & 0x07; extra = 3; }
+                        else { i++; continue; }
+                        if (i + extra >= s.size()) break;
+                        for (size_t j = 1; j <= extra; ++j) cp = (cp << 6) | (s[i + j] & 0x3F);
+                        cps.push_back(cp); i += extra + 1;
+                    }
+                    return cps;
+                };
+                bool needFallback = false;
+                FPDF_FONT objFont = FPDFTextObj_GetFont(best);
+                if (!fallbackFont.empty() && objFont) {
+                    for (uint32_t cp : decodeUtf8(newText)) {
+                        if (cp == ' ' || cp == '\t' || cp == '\n' || cp == '\r') continue;
+                        float w = 0.0f;
+                        if (!FPDFFont_GetGlyphWidth(objFont, cp, static_cast<float>(fontSize), &w) || w <= 0.0f) {
+                            needFallback = true;
+                            break;
+                        }
+                    }
+                }
+
+                FPDF_PAGEOBJECT target = best;
+                auto utf16 = utf8_to_utf16le(newText);
+
+                if (needFallback) {
+                    // Original font can't render the new glyphs — recreate in a
+                    // standard font at the same position (Acrobat-style substitution).
+                    spdlog::info("edit_text: glyph coverage gap, substituting font '{}'", fallbackFont);
+                    FPDFPage_RemoveObject(page, best);
+                    FPDFPageObj_Destroy(best);
+                    FPDF_FONT font = FPDFText_LoadStandardFont(doc_, fallbackFont.c_str());
+                    if (!font) font = FPDFText_LoadStandardFont(doc_, "Helvetica");
+                    target = FPDFPageObj_CreateTextObj(doc_, font, static_cast<float>(fontSize));
+                    if (!target) {
+                        FPDF_ClosePage(page);
+                        return std::unexpected(EngineError::Unknown);
+                    }
+                    unsigned int r = 0, g = 0, b = 0;
+                    parseHexColor(color, r, g, b);
+                    FPDFPageObj_SetFillColor(target, r, g, b, 255);
+                    if (!FPDFText_SetText(target, reinterpret_cast<FPDF_WIDESTRING>(utf16.data()))) {
+                        FPDFPageObj_Destroy(target);
+                        FPDF_ClosePage(page);
+                        return std::unexpected(EngineError::Unknown);
+                    }
+                    FPDFPageObj_Transform(target, 1.0, 0.0, 0.0, 1.0, origLeft, origBottom);
+                    FPDFPage_InsertObject(page, target);
+                } else {
+                    if (hasColor) {
+                        unsigned int r = 0, g = 0, b = 0;
+                        parseHexColor(color, r, g, b);
+                        FPDFPageObj_SetFillColor(target, r, g, b, 255);
+                    }
+                    if (!FPDFText_SetText(target, reinterpret_cast<FPDF_WIDESTRING>(utf16.data()))) {
+                        spdlog::error("edit_text: FPDFText_SetText failed on the existing object");
+                        FPDF_ClosePage(page);
+                        return std::unexpected(EngineError::Unknown);
+                    }
+                }
+
+                // Regenerate so the new text's bounds are accurate, then squeeze to fit.
+                if (!FPDFPage_GenerateContent(page)) {
+                    spdlog::error("edit_text: FPDFPage_GenerateContent failed");
+                    FPDF_ClosePage(page);
+                    return std::unexpected(EngineError::Unknown);
+                }
+                if (axisAligned && origWidth > 0.0) {
+                    float nl = 0, nb = 0, nr = 0, nt = 0;
+                    if (FPDFPageObj_GetBounds(target, &nl, &nb, &nr, &nt)) {
+                        const double newWidth = static_cast<double>(nr - nl);
+                        if (newWidth > origWidth && newWidth > 0.0) {
+                            const double scaleX = origWidth / newWidth;
+                            // Horizontal compression about the original left edge —
+                            // keeps the line start, baseline, and font size, never overflows.
+                            FPDFPageObj_Transform(target, scaleX, 0.0, 0.0, 1.0,
+                                                  origLeft * (1.0 - scaleX), 0.0);
+                            if (!FPDFPage_GenerateContent(page)) {
+                                spdlog::error("edit_text: FPDFPage_GenerateContent failed after squeeze");
+                                FPDF_ClosePage(page);
+                                return std::unexpected(EngineError::Unknown);
+                            }
+                        }
+                    }
+                }
+
                 FPDF_ClosePage(page);
             } else if (type == "update_field") {
                 if (!op.contains("data") || !op["data"].is_object()) {
@@ -1812,43 +2036,100 @@ std::expected<void, EngineError> PdfiumDocument::applyEdits(const std::string& e
                     return std::unexpected(EngineError::InvalidFormat);
                 }
                 auto data = op["data"];
-                std::string value;
-                if (data["value"].is_boolean()) {
-                    value = data["value"].get<bool>() ? "Yes" : "Off";
-                } else if (data["value"].is_string()) {
-                    value = data["value"].get<std::string>();
-                } else {
+                const bool isBool = data["value"].is_boolean();
+                const bool boolVal = isBool && data["value"].get<bool>();
+                std::string strVal;
+                if (data["value"].is_string()) strVal = data["value"].get<std::string>();
+                else if (!isBool) {
                     spdlog::error("update_field value must be a string or boolean");
                     return std::unexpected(EngineError::InvalidFormat);
                 }
 
-                std::string id = op.value("id", "");
-                int annotIndex = -1;
-                size_t lastUnderscore = id.find_last_of('_');
-                if (lastUnderscore != std::string::npos) {
-                    try {
-                        annotIndex = std::stoi(id.substr(lastUnderscore + 1));
-                    } catch (...) {
-                        annotIndex = -1;
-                    }
-                }
+                // Field identity: prefer data.annotationId (NM or anno_<page>_<idx>),
+                // fall back to the op id (legacy) for compatibility.
+                std::string targetId = data.value("annotationId", op.value("id", ""));
 
-                if (annotIndex >= 0) {
                 FPDF_PAGE page = FPDF_LoadPage(doc_, pageIndex);
                 if (!page) {
-                        spdlog::error("Failed to load page index {} for field update", pageIndex);
+                    spdlog::error("Failed to load page index {} for update_field", pageIndex);
                     return std::unexpected(EngineError::Unknown);
                 }
-                    FPDF_ANNOTATION annot = FPDFPage_GetAnnot(page, annotIndex);
-                    if (annot) {
-                        auto utf16 = utf8_to_utf16le(value);
-                        FPDFAnnot_SetStringValue(annot, "V", reinterpret_cast<FPDF_WIDESTRING>(utf16.data()));
 
-                        FPDFPage_GenerateContent(page);
+                FPDF_FORMFILLINFO formInfo{};
+                formInfo.version = 2;
+                FPDF_FORMHANDLE form = FPDFDOC_InitFormFillEnvironment(doc_, &formInfo);
+
+                int count = FPDFPage_GetAnnotCount(page);
+                FPDF_ANNOTATION target = nullptr;
+                for (int i = 0; i < count; ++i) {
+                    FPDF_ANNOTATION annot = FPDFPage_GetAnnot(page, i);
+                    if (!annot) continue;
+                    std::string id;
+                    unsigned long len = FPDFAnnot_GetStringValue(annot, "NM", nullptr, 0);
+                    if (len > 2) {
+                        std::vector<uint8_t> buf(len);
+                        FPDFAnnot_GetStringValue(annot, "NM", reinterpret_cast<FPDF_WCHAR*>(buf.data()), len);
+                        id = utf16le_to_utf8(reinterpret_cast<const char16_t*>(buf.data()), len / sizeof(char16_t));
+                        while (!id.empty() && id.back() == '\0') id.pop_back();
+                    }
+                    if (id.empty()) id = "anno_" + std::to_string(pageIndex) + "_" + std::to_string(i);
+                    if (id == targetId) { target = annot; break; }
                     FPDFPage_CloseAnnot(annot);
                 }
-                    FPDF_ClosePage(page);
+
+                if (target) {
+                    int fieldType = form ? FPDFAnnot_GetFormFieldType(form, target) : -1;
+                    if (form) FORM_OnAfterLoadPage(page, form);
+
+                    if (fieldType == 2 || fieldType == 3) {
+                        // Checkbox / radio: /V + /AS (predefined on/off appearances render directly).
+                        // NB: "Yes" is the common on-state; custom export values are a known v1 limitation.
+                        std::string state = boolVal ? "Yes" : "Off";
+                        auto u = utf8_to_utf16le(state);
+                        FPDFAnnot_SetStringValue(target, "V", reinterpret_cast<FPDF_WIDESTRING>(u.data()));
+                        FPDFAnnot_SetStringValue(target, "AS", reinterpret_cast<FPDF_WIDESTRING>(u.data()));
+                    } else if (form && FORM_SetFocusedAnnot(form, target)) {
+                        if (fieldType == 4 || fieldType == 5) {
+                            // Choice (combo/listbox): select the option whose label matches the value.
+                            int optCount = FPDFAnnot_GetOptionCount(form, target);
+                            int sel = -1;
+                            for (int o = 0; o < optCount; ++o) {
+                                unsigned long ol = FPDFAnnot_GetOptionLabel(form, target, o, nullptr, 0);
+                                if (ol <= 2) continue;
+                                std::vector<FPDF_WCHAR> ob(ol / 2);
+                                FPDFAnnot_GetOptionLabel(form, target, o, ob.data(), ol);
+                                std::string label = utf16le_to_utf8(reinterpret_cast<const char16_t*>(ob.data()), ob.size());
+                                while (!label.empty() && label.back() == '\0') label.pop_back();
+                                if (label == strVal) { sel = o; break; }
                             }
+                            if (sel >= 0) {
+                                FORM_SetIndexSelected(form, page, sel, 1);
+                            } else {
+                                auto u = utf8_to_utf16le(strVal);
+                                FPDFAnnot_SetStringValue(target, "V", reinterpret_cast<FPDF_WIDESTRING>(u.data()));
+                            }
+                        } else {
+                            // Text field: select-all + replace → form module regenerates the appearance.
+                            FORM_SelectAllText(form, page);
+                            auto u = utf8_to_utf16le(strVal);
+                            FORM_ReplaceSelection(form, page, reinterpret_cast<FPDF_WIDESTRING>(u.data()));
+                        }
+                        FORM_ForceToKillFocus(form);
+                    } else {
+                        // Fallback (no form env): best-effort value set.
+                        std::string v = isBool ? (boolVal ? "Yes" : "Off") : strVal;
+                        auto u = utf8_to_utf16le(v);
+                        FPDFAnnot_SetStringValue(target, "V", reinterpret_cast<FPDF_WIDESTRING>(u.data()));
+                    }
+
+                    if (form) FORM_OnBeforeClosePage(page, form);
+                    FPDFPage_CloseAnnot(target);
+                } else {
+                    spdlog::warn("update_field: field '{}' not found on page {}", targetId, pageIndex);
+                }
+
+                if (form) FPDFDOC_ExitFormFillEnvironment(form);
+                FPDF_ClosePage(page);
             } else if (type == "image_overlay") {
                 if (!op.contains("data") || !op["data"].is_object()) {
                     spdlog::error("image_overlay operation missing 'data' object");

engine/src/parser/pdfium_document.hpp

@@ -16,6 +16,8 @@ namespace pdfengine::fonts::loader { class FontResolver; }
 
 namespace pdfengine::parser {
 
+class PdfiumDocument; // a page keeps its owning document alive (see ownerDoc_)
+
 #ifdef PDFENGINE_WITH_PDFIUM
 using NativeDocHandle = FPDF_DOCUMENT;
 using NativePageHandle = FPDF_PAGE;
@@ -28,7 +30,8 @@ using NativeTextHandle = void*;
 
 class PdfiumPage : public PdfPage {
 public:
-    PdfiumPage(NativeDocHandle docHandle, NativePageHandle pageHandle, int pageIndex);
+    PdfiumPage(NativeDocHandle docHandle, NativePageHandle pageHandle, int pageIndex,
+               std::shared_ptr<PdfiumDocument> owner = nullptr);
     ~PdfiumPage() override;
 
     PdfiumPage(const PdfiumPage&) = delete;
@@ -58,6 +61,10 @@ private:
     mutable NativeTextHandle textPage_ = nullptr;
     int pageIndex_ = 0;
     mutable std::mutex textMutex_;
+    // Keeps the owning document (and thus the native FPDF_DOCUMENT) alive for as
+    // long as this page exists, so page_/textPage_ can never dangle. Declared
+    // here (destroyed last) so it outlives the handle teardown in the destructor.
+    std::shared_ptr<PdfiumDocument> ownerDoc_;
 #ifdef PDFENGINE_WITH_PDFIUM
     mutable std::unordered_map<std::string, FPDF_FONT> fontHandleCache_;
 #endif
@@ -103,7 +110,10 @@ private:
     mutable std::unordered_map<std::string, std::vector<uint8_t>> fontDataCache_;
     mutable int fontDataScannedPages_ = 0;
 
-    mutable std::unordered_map<int, std::shared_ptr<PdfPage>> pageCache_;
+    // Weak so the document never co-owns its pages: ownership runs page → document
+    // only. A cached entry is reused while a page is still referenced elsewhere,
+    // and lazily rebuilt once it expires.
+    mutable std::unordered_map<int, std::weak_ptr<PdfPage>> pageCache_;
     mutable std::mutex pageCacheMutex_;
 
     // Font Engine Bridge

engine/src/text/selection.cpp

@@ -0,0 +1,349 @@
+// Adobe-grade hit-testing & text selection for PdfPage.
+//
+// Concrete implementations of PdfPage::orderedGlyphs / hitGlyph / selectRange,
+// built on top of the (virtual) extractTextWithBounds(). All maths is in
+// page-point space with a top-left origin — identical to the frontend
+// TextSelectionModel, so engine and browser selections agree.
+
+#include "pdfengine/pdf_document.hpp"
+
+#include <algorithm>
+#include <cctype>
+#include <cmath>
+#include <limits>
+#include <map>
+
+namespace pdfengine {
+namespace {
+
+struct OGlyph {
+    GlyphBounds g;
+    int line = 0;
+    double right = 0.0;
+    double bottom = 0.0;
+    double mid = 0.0; // horizontal centre
+};
+
+struct LineBand {
+    double top = 0.0;
+    double bottom = 0.0;
+    double mid = 0.0; // vertical centre of the first glyph
+    int start = 0;    // inclusive glyph index
+    int end = 0;      // exclusive glyph index
+};
+
+// Uniform 2D grid over glyph bounding boxes — a true spatial index for the
+// point→glyph query, independent of line assignment. This is what makes
+// hit-testing exact on overlapping/dense content (diacritics, multi-column,
+// rotated runs) where a line-bucketed search would miss.
+struct Grid {
+    double minX = 0.0, minY = 0.0, cell = 1.0;
+    int cols = 0, rows = 0;
+    std::vector<std::vector<int>> cells; // size cols*rows; glyph indices per cell
+
+    [[nodiscard]] bool empty() const { return cols == 0 || rows == 0; }
+    [[nodiscard]] int at(int c, int r) const { return r * cols + c; }
+};
+
+struct SelIndex {
+    std::vector<OGlyph> glyphs; // reading order
+    std::vector<LineBand> lines;
+    Grid grid;
+};
+
+bool isSpace(const std::string& t) {
+    if (t.empty()) return true;
+    for (unsigned char c : t)
+        if (!std::isspace(c)) return false;
+    return true;
+}
+
+// Cluster raw glyphs into lines (by vertical overlap) then sort each line L→R,
+// flattening into a single reading-order array with cached line bands.
+SelIndex buildIndex(const std::vector<GlyphBounds>& raw) {
+    SelIndex idx;
+    std::vector<GlyphBounds> clean;
+    clean.reserve(raw.size());
+    for (const auto& g : raw)
+        if (g.w >= 0 && g.h > 0) clean.push_back(g);
+    if (clean.empty()) return idx;
+
+    std::sort(clean.begin(), clean.end(), [](const GlyphBounds& a, const GlyphBounds& b) {
+        return (a.y + a.h / 2) < (b.y + b.h / 2);
+    });
+
+    std::vector<std::vector<GlyphBounds>> rows;
+    for (const auto& g : clean) {
+        const double gMid = g.y + g.h / 2;
+        bool placed = false;
+        if (!rows.empty()) {
+            auto& row = rows.back();
+            double top = std::numeric_limits<double>::max();
+            double bottom = std::numeric_limits<double>::lowest();
+            for (const auto& r : row) {
+                top = std::min(top, r.y);
+                bottom = std::max(bottom, r.y + r.h);
+            }
+            const double tol = g.h * 0.25;
+            if (gMid >= top - tol && gMid <= bottom + tol) {
+                row.push_back(g);
+                placed = true;
+            }
+        }
+        if (!placed) rows.push_back({g});
+    }
+
+    int running = 0;
+    for (auto& row : rows) {
+        std::sort(row.begin(), row.end(),
+                  [](const GlyphBounds& a, const GlyphBounds& b) { return a.x < b.x; });
+        LineBand band;
+        band.start = running;
+        const int lineNo = static_cast<int>(idx.lines.size());
+        double top = std::numeric_limits<double>::max();
+        double bottom = std::numeric_limits<double>::lowest();
+        for (const auto& g : row) {
+            OGlyph og;
+            og.g = g;
+            og.line = lineNo;
+            og.right = g.x + g.w;
+            og.bottom = g.y + g.h;
+            og.mid = g.x + g.w / 2;
+            idx.glyphs.push_back(og);
+            top = std::min(top, g.y);
+            bottom = std::max(bottom, g.y + g.h);
+            running++;
+        }
+        band.end = running;
+        band.top = top;
+        band.bottom = bottom;
+        band.mid = row.front().y + row.front().h / 2;
+        idx.lines.push_back(band);
+    }
+
+    // Build the spatial grid over the final glyph set.
+    Grid& grid = idx.grid;
+    double minX = std::numeric_limits<double>::max();
+    double minY = std::numeric_limits<double>::max();
+    double maxX = std::numeric_limits<double>::lowest();
+    double maxY = std::numeric_limits<double>::lowest();
+    double sumH = 0.0;
+    for (const auto& og : idx.glyphs) {
+        minX = std::min(minX, og.g.x);
+        minY = std::min(minY, og.g.y);
+        maxX = std::max(maxX, og.right);
+        maxY = std::max(maxY, og.bottom);
+        sumH += og.g.h;
+    }
+    // Cell ≈ average glyph height (~one text line), so most cells hold a handful
+    // of glyphs. Grow the cell if the grid would otherwise be unreasonably large.
+    double cell = std::max(1.0, sumH / static_cast<double>(idx.glyphs.size()));
+    auto dim = [&](double lo, double hi) {
+        return std::max(1, static_cast<int>((hi - lo) / cell) + 1);
+    };
+    int gCols = dim(minX, maxX);
+    int gRows = dim(minY, maxY);
+    while (static_cast<long long>(gCols) * gRows > 2'000'000 && cell < 1e9) {
+        cell *= 2.0;
+        gCols = dim(minX, maxX);
+        gRows = dim(minY, maxY);
+    }
+    grid.minX = minX;
+    grid.minY = minY;
+    grid.cell = cell;
+    grid.cols = gCols;
+    grid.rows = gRows;
+    grid.cells.assign(static_cast<size_t>(gCols) * gRows, {});
+    auto col = [&](double x) { return std::clamp(static_cast<int>((x - minX) / cell), 0, gCols - 1); };
+    auto rowOf = [&](double y) { return std::clamp(static_cast<int>((y - minY) / cell), 0, gRows - 1); };
+    for (int i = 0; i < static_cast<int>(idx.glyphs.size()); ++i) {
+        const OGlyph& og = idx.glyphs[i];
+        const int c0 = col(og.g.x), c1 = col(og.right);
+        const int r0 = rowOf(og.g.y), r1 = rowOf(og.bottom);
+        for (int r = r0; r <= r1; ++r)
+            for (int c = c0; c <= c1; ++c)
+                grid.cells[grid.at(c, r)].push_back(i);
+    }
+    return idx;
+}
+
+// --- spatial index queries (Adobe-grade, fast on dense/overlapping content) --
+//
+// Point→glyph uses the 2D grid (exact regardless of line layout). Caret/line
+// positioning uses the line bands via binary search (lines are in ascending
+// vertical order; each line's glyphs are contiguous and x-sorted). Both stay
+// cheap on pages with tens of thousands of glyphs.
+
+// Exact glyph under a point via the spatial grid — never misses a containing
+// glyph, even where text lines overlap.
+int glyphAt(const SelIndex& idx, double x, double y) {
+    const Grid& grid = idx.grid;
+    if (grid.empty() || x < grid.minX || y < grid.minY) return -1;
+    const int c = static_cast<int>((x - grid.minX) / grid.cell);
+    const int r = static_cast<int>((y - grid.minY) / grid.cell);
+    if (c < 0 || c >= grid.cols || r < 0 || r >= grid.rows) return -1;
+    for (const int i : grid.cells[grid.at(c, r)]) {
+        const OGlyph& g = idx.glyphs[i];
+        if (x >= g.g.x && x <= g.right && y >= g.g.y && y <= g.bottom) return i;
+    }
+    return -1;
+}
+
+int lineAt(const SelIndex& idx, double y) {
+    const auto& lines = idx.lines;
+    if (lines.empty()) return -1;
+
+    // Binary search for the last line whose top <= y.
+    int lo = 0;
+    int hi = static_cast<int>(lines.size());
+    while (lo < hi) {
+        const int m = (lo + hi) / 2;
+        if (lines[m].top <= y) lo = m + 1;
+        else hi = m;
+    }
+    const int cand = lo - 1; // -1 when y is above every line
+
+    // Check the immediate neighbourhood for true containment, else take the
+    // nearest band by centre. Bands can overlap slightly (sub/superscript
+    // tolerance), so a ±1 window around the search boundary is enough.
+    int best = -1;
+    double bestDist = std::numeric_limits<double>::max();
+    for (int i = cand - 1; i <= cand + 1; ++i) {
+        if (i < 0 || i >= static_cast<int>(lines.size())) continue;
+        if (y >= lines[i].top && y <= lines[i].bottom) return i;
+        const double d = std::abs(y - lines[i].mid);
+        if (d < bestDist) {
+            bestDist = d;
+            best = i;
+        }
+    }
+    if (best >= 0) return best;
+    return cand < 0 ? 0 : static_cast<int>(lines.size()) - 1;
+}
+
+// Index of the last glyph in [start,end) whose left edge x <= queryX, or
+// start-1 if queryX is left of the whole line. Glyphs are sorted by x.
+static int lastGlyphLeftOf(const SelIndex& idx, const LineBand& line, double x) {
+    int lo = line.start;
+    int hi = line.end;
+    while (lo < hi) {
+        const int m = (lo + hi) / 2;
+        if (idx.glyphs[m].g.x <= x) lo = m + 1;
+        else hi = m;
+    }
+    return lo - 1;
+}
+
+int caretAt(const SelIndex& idx, double x, double y) {
+    // If the point lands on a glyph, the caret sits on its near or far side.
+    const int hit = glyphAt(idx, x, y);
+    if (hit >= 0) {
+        const OGlyph& g = idx.glyphs[hit];
+        return x < g.mid ? hit : hit + 1;
+    }
+    // Otherwise position within the nearest line (gaps / between lines / margins).
+    const int li = lineAt(idx, y);
+    if (li < 0) return 0;
+    const LineBand& line = idx.lines[li];
+    if (x <= idx.glyphs[line.start].g.x) return line.start;
+    if (x >= idx.glyphs[line.end - 1].right) return line.end;
+
+    const int cand = lastGlyphLeftOf(idx, line, x);
+    if (cand < line.start) return line.start;
+    const OGlyph& g = idx.glyphs[cand];
+    if (x <= g.right) return x < g.mid ? cand : cand + 1; // inside the glyph
+    return cand + 1;                                      // in the gap after it
+}
+
+std::string textOfRange(const SelIndex& idx, int start, int end) {
+    std::string out;
+    const OGlyph* prev = nullptr;
+    for (int i = start; i < end; ++i) {
+        const OGlyph& g = idx.glyphs[i];
+        if (prev) {
+            if (g.line != prev->line) {
+                out += '\n';
+            } else {
+                const double gap = g.g.x - prev->right;
+                if (!isSpace(g.g.text) && !isSpace(prev->g.text) && gap > g.g.h * 0.25)
+                    out += ' ';
+            }
+        }
+        out += g.g.text;
+        prev = &g;
+    }
+    return out;
+}
+
+std::vector<GlyphBounds> rectsOfRange(const SelIndex& idx, int start, int end) {
+    std::vector<GlyphBounds> rects;
+    if (start >= end) return rects;
+    // Group selected glyphs by line, preserving first-seen order.
+    std::map<int, std::pair<double, double>> spanByLine; // line -> {minX, maxRight}
+    std::vector<int> order;
+    for (int i = start; i < end; ++i) {
+        const OGlyph& g = idx.glyphs[i];
+        auto it = spanByLine.find(g.line);
+        if (it == spanByLine.end()) {
+            spanByLine[g.line] = {g.g.x, g.right};
+            order.push_back(g.line);
+        } else {
+            it->second.first = std::min(it->second.first, g.g.x);
+            it->second.second = std::max(it->second.second, g.right);
+        }
+    }
+    for (int line : order) {
+        const auto& span = spanByLine[line];
+        const LineBand& band = idx.lines[line];
+        GlyphBounds r;
+        r.text = "";
+        r.x = span.first;
+        r.y = band.top;
+        r.w = span.second - span.first;
+        r.h = band.bottom - band.top;
+        r.fontSize = 0.0;
+        rects.push_back(r);
+    }
+    return rects;
+}
+
+} // namespace
+
+std::expected<std::vector<GlyphBounds>, EngineError> PdfPage::orderedGlyphs() const {
+    auto raw = extractTextWithBounds();
+    if (!raw) return std::unexpected(raw.error());
+    const SelIndex idx = buildIndex(*raw);
+    std::vector<GlyphBounds> out;
+    out.reserve(idx.glyphs.size());
+    for (const auto& og : idx.glyphs) out.push_back(og.g);
+    return out;
+}
+
+std::expected<HitResult, EngineError> PdfPage::hitGlyph(double x, double y) const {
+    auto raw = extractTextWithBounds();
+    if (!raw) return std::unexpected(raw.error());
+    const SelIndex idx = buildIndex(*raw);
+    HitResult hit;
+    hit.line = lineAt(idx, y);
+    hit.caret = caretAt(idx, x, y);
+    hit.glyphIndex = glyphAt(idx, x, y);
+    return hit;
+}
+
+std::expected<TextSelection, EngineError>
+PdfPage::selectRange(double ax, double ay, double bx, double by) const {
+    auto raw = extractTextWithBounds();
+    if (!raw) return std::unexpected(raw.error());
+    const SelIndex idx = buildIndex(*raw);
+    int a = caretAt(idx, ax, ay);
+    int b = caretAt(idx, bx, by);
+    if (a > b) std::swap(a, b);
+    TextSelection sel;
+    sel.startGlyph = a;
+    sel.endGlyph = b;
+    sel.text = textOfRange(idx, a, b);
+    sel.rects = rectsOfRange(idx, a, b);
+    return sel;
+}
+
+} // namespace pdfengine

engine/tests/fonts_test.cpp

@@ -12,6 +12,8 @@
 #include "fonts/pdf_fonts/font_subset.hpp"
 
 #include <gtest/gtest.h>
+#include <algorithm>
+#include <cctype>
 #include <filesystem>
 #include <fstream>
 #include <iostream>
@@ -20,6 +22,19 @@
 
 namespace {
 
+// Case-insensitive substring check. System font filenames differ in case
+// across platforms (e.g. macOS ships "Times.ttc", Windows "times.ttf"), so the
+// font-fallback assertions match without regard to case.
+bool containsCI(const std::string& haystack, const std::string& needle) {
+    auto it = std::search(
+        haystack.begin(), haystack.end(), needle.begin(), needle.end(),
+        [](char a, char b) {
+            return std::tolower(static_cast<unsigned char>(a)) ==
+                   std::tolower(static_cast<unsigned char>(b));
+        });
+    return it != haystack.end();
+}
+
 bool saveGlyphAsPGM(const pdfengine::fonts::GlyphBitmap& bitmap, const std::string& filename) {
     if (bitmap.width == 0 || bitmap.height == 0 || bitmap.pixels.empty()) {
         return false;
@@ -1020,22 +1035,36 @@ TEST(FontFallbackTest, SingletonInstanceIsUnique) {
     EXPECT_EQ(&instance1, &instance2);
 }
 
-TEST(FontFallbackTest, StandardFontFallbacksOnWindows) {
+TEST(FontFallbackTest, StandardFontFallbacks) {
     using namespace pdfengine::fonts::pdf_fonts;
 
     auto& fallback = FontFallback::getInstance();
 
-    // Test Helvetica to Arial
+    // Helvetica resolves to its sans-serif substitute for the host platform.
     std::string path1 = fallback.getFallbackFontPath("Helvetica");
     EXPECT_FALSE(path1.empty());
     EXPECT_TRUE(std::filesystem::exists(path1));
-    EXPECT_TRUE(path1.find("arial") != std::string::npos || path1.find("ARIAL") != std::string::npos);
+#if defined(_WIN32)
+    EXPECT_TRUE(containsCI(path1, "arial") || containsCI(path1, "liberationsans"));
+#elif defined(__APPLE__)
+    // Arial may not be installed; the resolver then falls back to Helvetica.
+    EXPECT_TRUE(containsCI(path1, "arial") || containsCI(path1, "helvetica") ||
+                containsCI(path1, "liberationsans"));
+#else
+    EXPECT_TRUE(containsCI(path1, "liberationsans") || containsCI(path1, "dejavusans"));
+#endif
 
-    // Test Times to Times New Roman
+    // Times resolves to its serif substitute for the host platform.
     std::string path2 = fallback.getFallbackFontPath("Times-Roman");
     EXPECT_FALSE(path2.empty());
     EXPECT_TRUE(std::filesystem::exists(path2));
-    EXPECT_TRUE(path2.find("times") != std::string::npos || path2.find("TIMES") != std::string::npos);
+#if defined(_WIN32)
+    EXPECT_TRUE(containsCI(path2, "times") || containsCI(path2, "liberationserif"));
+#elif defined(__APPLE__)
+    EXPECT_TRUE(containsCI(path2, "times") || containsCI(path2, "liberationserif"));
+#else
+    EXPECT_TRUE(containsCI(path2, "liberationserif") || containsCI(path2, "dejavuserif"));
+#endif
 }
 
 TEST(FontFallbackTest, StyleModifierResolutions) {
@@ -1043,13 +1072,23 @@ TEST(FontFallbackTest, StyleModifierResolutions) {
 
     auto& fallback = FontFallback::getInstance();
 
-    // Bold Helvetica should map to Arial Bold
+    // Bold Helvetica should map to a bold sans-serif substitute.
     std::string pathBold = fallback.getFallbackFontPath("Helvetica", true, false);
-    EXPECT_TRUE(pathBold.find("arialbd") != std::string::npos);
+    EXPECT_FALSE(pathBold.empty());
+    EXPECT_TRUE(std::filesystem::exists(pathBold));
+#if defined(_WIN32)
+    // Windows ships the styled variants, so assert the exact bold face.
+    EXPECT_TRUE(containsCI(pathBold, "arialbd") || containsCI(pathBold, "liberationsans-bold"));
+#endif
 
-    // Bold Italic Times should map to Times New Roman Bold Italic
+    // Bold-italic Times should map to a bold-italic serif substitute.
     std::string pathBoldItalic = fallback.getFallbackFontPath("Times", true, true);
-    EXPECT_TRUE(pathBoldItalic.find("timesbi") != std::string::npos);
+    EXPECT_FALSE(pathBoldItalic.empty());
+    EXPECT_TRUE(std::filesystem::exists(pathBoldItalic));
+#if defined(_WIN32)
+    EXPECT_TRUE(containsCI(pathBoldItalic, "timesbi") ||
+                containsCI(pathBoldItalic, "liberationserif-bolditalic"));
+#endif
 }
 
 TEST(FontFallbackTest, CustomFallbackRegistration) {
@@ -1058,19 +1097,26 @@ TEST(FontFallbackTest, CustomFallbackRegistration) {
     auto& fallback = FontFallback::getInstance();
     fallback.resetToDefaults();
 
-    // Lookup standard Arial path
+    // Lookup the default substitute path for Helvetica.
     std::string standardPath = fallback.getFallbackFontPath("Helvetica");
 
-    // Register custom override for "helvetica" pointing to times.ttf
-    fallback.registerFallback("helvetica", "C:\\Windows\\Fonts\\times.ttf");
+    // The resolver only returns an override whose file actually exists on disk,
+    // so register a real temp file rather than a hardcoded OS-specific path.
+    std::filesystem::path overrideFont =
+        std::filesystem::temp_directory_path() / "pdfengine_custom_fallback.ttf";
+    { std::ofstream(overrideFont) << "stub-font"; }
+
+    fallback.registerFallback("helvetica", overrideFont.string());
 
     std::string overridenPath = fallback.getFallbackFontPath("Helvetica");
-    EXPECT_EQ(overridenPath, "C:\\Windows\\Fonts\\times.ttf");
+    EXPECT_EQ(overridenPath, overrideFont.string());
 
-    // Reset back to defaults
+    // Reset back to defaults and confirm the original substitute returns.
     fallback.resetToDefaults();
     std::string restoredPath = fallback.getFallbackFontPath("Helvetica");
     EXPECT_EQ(restoredPath, standardPath);
+
+    std::filesystem::remove(overrideFont);
 }
 
 TEST(FontSubsetTest, SubsetTagParsingAndStripping) {

frontend/src/App.tsx

@@ -12,6 +12,7 @@ import type { CustomConfirmationOptions } from './components/custom/CustomConfir
 import { PDFViewer } from './viewer/PDFViewer';
 import type { PDFViewerRef } from './viewer/PDFViewer';
 import type { Annotation } from './viewer/AnnotationLayer';
+import type { EditableRun } from './viewer/TextEditLayer';
 import { gatewayService } from './lib/gatewayService';
 import type { DocumentInfo, SearchResult, EditOperation, DocumentMetadata, FontInfo, OutlineItem } from './lib/gatewayService';
 import { viewportRectToPdf } from './lib/coordinateMapping';
@@ -271,6 +272,19 @@ function App() {
     setActiveTool('select');
   };
 
+  // Rewrite existing page text in place. `run` is in PDF bottom-left points (the
+  // space getPageModel / the engine use), so no coordinate flip is needed here.
+  const handleEditText = (pageIndex: number, run: EditableRun, newText: string) => {
+    applyOps([{
+      id: rid('edit'), type: 'edit_text', pageIndex,
+      data: {
+        x: run.x, y: run.y, width: run.w, height: run.h,
+        newText, originalText: run.text, fontSize: run.fontSize, fallbackFont: 'Helvetica',
+      },
+    }], 'Text updated');
+    setActiveTool('select');
+  };
+
   const handlePlaceStamp = (pageIndex: number, point: { x: number; y: number }) => {
     if (!activeStamp) return;
     const fontSize = 22;
@@ -523,13 +537,14 @@ function App() {
                 searchCurrentMatch={searchCurrentMatch}
                 onAnnotationAdded={handleAnnotationAdded}
                 onAnnotationUpdate={handleUpdateAnnotation}
-                onAnnotationClick={(a) => {
+                onAnnotationClick={() => {
                   setInspectorTab('notes');
                   if (!isInspectorOpen) setIsInspectorOpen(true);
                 }}
                 onPageVisible={setCurrentPage}
                 onRedactArea={handleRedactArea}
                 onPlaceText={handlePlaceText}
+                onEditText={handleEditText}
                 onPlaceStamp={handlePlaceStamp}
                 onPlaceSignature={handlePlaceSignature}
               />

frontend/src/components/ToolRail.tsx

@@ -17,6 +17,17 @@ const TOOLS: (ToolDef | 'divider')[] = [
   { id: 'draw', label: 'Draw (ink)', shortcut: 'D', icon: <DrawIcon /> },
   { id: 'comment', label: 'Comment', shortcut: 'C', icon: <CommentIcon /> },
   { id: 'textbox', label: 'Text box', shortcut: 'T', icon: <TextBoxIcon /> },
+  {
+    id: 'edit_text',
+    label: 'Edit text',
+    shortcut: 'E',
+    icon: (
+      <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth={2} strokeLinecap="round" strokeLinejoin="round">
+        <path d="M12 20h9" />
+        <path d="M16.5 3.5a2.12 2.12 0 0 1 3 3L7 19l-4 1 1-4Z" />
+      </svg>
+    ),
+  },
   { id: 'signature', label: 'Signature', shortcut: 'S', icon: <SignatureIcon /> },
   { id: 'stamp', label: 'Stamp', shortcut: 'M', icon: <StampIcon /> },
   'divider',

frontend/src/components/Toolbar.tsx

@@ -25,6 +25,7 @@ const TOOL_META: Record<ToolId, { label: string; icon: React.ReactNode }> = {
   draw: { label: 'Draw', icon: <DrawIcon size={17} /> },
   comment: { label: 'Comment', icon: <CommentIcon size={17} /> },
   textbox: { label: 'Text box', icon: <TextBoxIcon size={17} /> },
+  edit_text: { label: 'Edit text', icon: <TextBoxIcon size={17} /> },
   signature: { label: 'Signature', icon: <SignatureIcon size={17} /> },
   stamp: { label: 'Stamp', icon: <StampIcon size={17} /> },
   redact: { label: 'Redact', icon: <RedactIcon size={17} /> },

frontend/src/lib/gatewayService.ts

@@ -185,8 +185,22 @@ export type EditOperationDataMap = {
   page_reorder: PageReorderData;
   delete_annotation: DeleteAnnotationData;
   update_annotation: UpdateAnnotationData;
+  edit_text: EditTextData;
 };
 
+export interface EditTextData {
+  // Target run bbox in PDF bottom-left page space (same space getPageModel returns).
+  x: number;
+  y: number;
+  width: number;
+  height: number;
+  newText: string;
+  originalText?: string;
+  fontSize?: number;
+  color?: string;
+  fallbackFont?: string;
+}
+
 export interface DeleteAnnotationData {
   annotationId: string;
 }

frontend/src/lib/textSelection.ts

@@ -0,0 +1,244 @@
+// Adobe-grade text-selection model.
+//
+// Operates purely in **page-point space** (unzoomed, top-left origin) — the same
+// space the engine emits glyph bounds in. Callers convert mouse coordinates into
+// this space (divide by zoom) before querying, and scale the returned rects back
+// up (multiply by zoom) for rendering.
+//
+// A *caret* is a position between glyphs, indexed 0..N (N = glyph count). A
+// selection is an ordered pair of carets {start, end}; start may be greater than
+// end for a backward (right-to-left) drag. All range helpers normalise
+// internally, so the component is free to keep anchor/focus unnormalised.
+
+import type { Glyph } from './gatewayService';
+
+export interface OrderedGlyph extends Glyph {
+  index: number; // position in reading order
+  line: number; // line band this glyph belongs to
+  right: number; // x + w (cached)
+  bottom: number; // y + h (cached)
+  mid: number; // x + w/2 (cached)
+}
+
+export interface CaretRange {
+  start: number;
+  end: number;
+}
+
+export interface SelRect {
+  x: number;
+  y: number;
+  w: number;
+  h: number;
+}
+
+interface LineBand {
+  top: number;
+  bottom: number;
+  mid: number;
+  start: number; // first glyph index (inclusive)
+  end: number; // last glyph index + 1 (exclusive)
+}
+
+const isSpace = (t: string) => t.length === 0 || /^\s+$/.test(t);
+
+export class TextSelectionModel {
+  readonly glyphs: OrderedGlyph[] = [];
+  private readonly lines: LineBand[] = [];
+
+  constructor(raw: Glyph[]) {
+    const clean = raw.filter((g) => g.w >= 0 && g.h > 0);
+    if (clean.length === 0) return;
+
+    // Cluster glyphs into lines by vertical overlap. Sort by vertical centre so
+    // rows arrive top-to-bottom even when the source order is arbitrary.
+    const byY = [...clean].sort((a, b) => a.y + a.h / 2 - (b.y + b.h / 2));
+    const rows: Glyph[][] = [];
+    for (const g of byY) {
+      const gMid = g.y + g.h / 2;
+      const row = rows[rows.length - 1];
+      if (row) {
+        // Band of the current row so far.
+        const top = Math.min(...row.map((r) => r.y));
+        const bottom = Math.max(...row.map((r) => r.y + r.h));
+        // Same line if the glyph centre falls inside the row band (with a small
+        // tolerance for sub/superscript jitter).
+        const tol = g.h * 0.25;
+        if (gMid >= top - tol && gMid <= bottom + tol) {
+          row.push(g);
+          continue;
+        }
+      }
+      rows.push([g]);
+    }
+
+    // Flatten rows (each sorted left→right) into a single reading-order array and
+    // record the line bands for spatial queries.
+    let idx = 0;
+    rows.forEach((row, lineNo) => {
+      row.sort((a, b) => a.x - b.x);
+      const start = idx;
+      for (const g of row) {
+        this.glyphs.push({
+          ...g,
+          index: idx,
+          line: lineNo,
+          right: g.x + g.w,
+          bottom: g.y + g.h,
+          mid: g.x + g.w / 2,
+        });
+        idx += 1;
+      }
+      this.lines.push({
+        top: Math.min(...row.map((r) => r.y)),
+        bottom: Math.max(...row.map((r) => r.y + r.h)),
+        mid: row[0].y + row[0].h / 2,
+        start,
+        end: idx,
+      });
+    });
+  }
+
+  get length(): number {
+    return this.glyphs.length;
+  }
+
+  /** Index of the line whose band contains y, else the nearest line by centre. */
+  private lineAt(y: number): number {
+    if (this.lines.length === 0) return -1;
+    for (let i = 0; i < this.lines.length; i++) {
+      if (y >= this.lines[i].top && y <= this.lines[i].bottom) return i;
+    }
+    // Above the first / below the last / in an inter-line gap → nearest centre.
+    let best = 0;
+    let bestDist = Infinity;
+    for (let i = 0; i < this.lines.length; i++) {
+      const d = Math.abs(y - this.lines[i].mid);
+      if (d < bestDist) {
+        bestDist = d;
+        best = i;
+      }
+    }
+    return best;
+  }
+
+  /** Nearest caret (0..N) to a point. */
+  caretAt(x: number, y: number): number {
+    const li = this.lineAt(y);
+    if (li < 0) return 0;
+    const line = this.lines[li];
+    if (x <= this.glyphs[line.start].x) return line.start;
+    if (x >= this.glyphs[line.end - 1].right) return line.end;
+    for (let i = line.start; i < line.end; i++) {
+      const g = this.glyphs[i];
+      if (x >= g.x && x <= g.right) return x < g.mid ? g.index : g.index + 1;
+      // In the gap before this glyph.
+      if (x < g.x) return g.index;
+    }
+    return line.end;
+  }
+
+  /** Glyph index directly under a point, or -1. */
+  glyphAt(x: number, y: number): number {
+    const li = this.lineAt(y);
+    if (li < 0) return -1;
+    const line = this.lines[li];
+    if (y < line.top || y > line.bottom) return -1;
+    for (let i = line.start; i < line.end; i++) {
+      const g = this.glyphs[i];
+      if (x >= g.x && x <= g.right) return i;
+    }
+    return -1;
+  }
+
+  /** Word caret-range around a point (run of non-space within one line). */
+  wordRangeAt(x: number, y: number): CaretRange | null {
+    let gi = this.glyphAt(x, y);
+    if (gi < 0) {
+      // Snap to the caret, then take the glyph just right of it (or left at EOL).
+      const c = this.caretAt(x, y);
+      gi = c < this.length ? c : c - 1;
+      if (gi < 0 || gi >= this.length) return null;
+    }
+    const line = this.glyphs[gi].line;
+    if (isSpace(this.glyphs[gi].text)) return { start: gi, end: gi + 1 };
+    let s = gi;
+    let e = gi;
+    while (s - 1 >= 0 && this.glyphs[s - 1].line === line && !isSpace(this.glyphs[s - 1].text)) s--;
+    while (e + 1 < this.length && this.glyphs[e + 1].line === line && !isSpace(this.glyphs[e + 1].text)) e++;
+    return { start: s, end: e + 1 };
+  }
+
+  /** Whole-line caret-range around a point. */
+  lineRangeAt(_x: number, y: number): CaretRange | null {
+    const li = this.lineAt(y);
+    if (li < 0) return null;
+    return { start: this.lines[li].start, end: this.lines[li].end };
+  }
+
+  selectAll(): CaretRange {
+    return { start: 0, end: this.length };
+  }
+
+  private static norm(r: CaretRange): CaretRange {
+    return r.start <= r.end ? r : { start: r.end, end: r.start };
+  }
+
+  glyphsInRange(r: CaretRange): OrderedGlyph[] {
+    const { start, end } = TextSelectionModel.norm(r);
+    return this.glyphs.slice(start, end);
+  }
+
+  /** Reconstructed text with intra-line spaces and inter-line newlines. */
+  textOfRange(r: CaretRange): string {
+    const sel = this.glyphsInRange(r);
+    if (sel.length === 0) return '';
+    let out = '';
+    let prev: OrderedGlyph | null = null;
+    for (const g of sel) {
+      if (prev) {
+        if (g.line !== prev.line) out += '\n';
+        else {
+          const gap = g.x - prev.right;
+          const alreadySpace = isSpace(g.text) || isSpace(prev.text);
+          if (!alreadySpace && gap > g.h * 0.25) out += ' ';
+        }
+      }
+      out += g.text;
+      prev = g;
+    }
+    return out.replace(/[ \t]+\n/g, '\n').trimEnd();
+  }
+
+  /** Per-line union rects covering the selection (page-point space). */
+  rectsOfRange(r: CaretRange): SelRect[] {
+    const sel = this.glyphsInRange(r);
+    if (sel.length === 0) return [];
+    const byLine = new Map<number, OrderedGlyph[]>();
+    for (const g of sel) {
+      const arr = byLine.get(g.line);
+      if (arr) arr.push(g);
+      else byLine.set(g.line, [g]);
+    }
+    const rects: SelRect[] = [];
+    for (const arr of byLine.values()) {
+      const x = Math.min(...arr.map((g) => g.x));
+      const right = Math.max(...arr.map((g) => g.right));
+      // Use the full line band height for a continuous highlight, not glyph height.
+      const band = this.lines[arr[0].line];
+      rects.push({ x, y: band.top, w: right - x, h: band.bottom - band.top });
+    }
+    return rects;
+  }
+
+  /** Tight union of all selection rects (page-point space) — for highlight bbox. */
+  unionRect(r: CaretRange): SelRect | null {
+    const rects = this.rectsOfRange(r);
+    if (rects.length === 0) return null;
+    const x = Math.min(...rects.map((q) => q.x));
+    const y = Math.min(...rects.map((q) => q.y));
+    const right = Math.max(...rects.map((q) => q.x + q.w));
+    const bottom = Math.max(...rects.map((q) => q.y + q.h));
+    return { x, y, w: right - x, h: bottom - y };
+  }
+}

frontend/src/lib/tools.ts

@@ -5,6 +5,7 @@ export type ToolId =
   | 'draw'
   | 'comment'
   | 'textbox'
+  | 'edit_text'
   | 'signature'
   | 'stamp'
   | 'redact';
@@ -34,6 +35,7 @@ export const TOOL_SHORTCUTS: Record<string, ToolId> = {
   d: 'draw',
   c: 'comment',
   t: 'textbox',
+  e: 'edit_text',
   s: 'signature',
   m: 'stamp',
   r: 'redact',

frontend/src/viewer/PDFViewer.tsx

@@ -4,6 +4,8 @@ import { SelectionLayer } from './SelectionLayer';
 import { AnnotationLayer } from './AnnotationLayer';
 import type { Annotation } from './AnnotationLayer';
 import { OverlayLayer } from './OverlayLayer';
+import { TextEditLayer } from './TextEditLayer';
+import type { EditableRun } from './TextEditLayer';
 import { SearchOverlayLayer } from './SearchOverlayLayer';
 import type { Rect } from '../lib/coordinateMapping';
 import { gatewayService } from '../lib/gatewayService';
@@ -33,6 +35,7 @@ interface PDFViewerProps {
   onPageVisible?: (pageIndex: number) => void;
   onRedactArea?: (pageIndex: number, bounds: Rect) => void;
   onPlaceText?: (pageIndex: number, rectPts: Rect, text: string) => void;
+  onEditText?: (pageIndex: number, run: EditableRun, newText: string) => void;
   onPlaceStamp?: (pageIndex: number, pointPts: { x: number; y: number }) => void;
   onPlaceSignature?: (pageIndex: number, pointPts: { x: number; y: number }) => void;
   onFieldChange?: (id: string, value: string | boolean, pageIndex: number) => void;
@@ -72,6 +75,7 @@ export const PDFViewer = React.forwardRef<PDFViewerRef, PDFViewerProps>(({
   onPageVisible,
   onRedactArea,
   onPlaceText,
+  onEditText,
   onPlaceStamp,
   onPlaceSignature,
   onFieldChange,
@@ -403,6 +407,7 @@ export const PDFViewer = React.forwardRef<PDFViewerRef, PDFViewerProps>(({
                       height={page.height}
                       zoom={zoom}
                       glyphs={pageTexts[page.index] || []}
+                      mode={activeTool === 'highlight' ? 'highlight' : 'select'}
                       onTextSelected={(text, bbox) => handleTextSelection(text, bbox, page.index)}
                     />
                   )}
@@ -436,6 +441,18 @@ export const PDFViewer = React.forwardRef<PDFViewerRef, PDFViewerProps>(({
                     onPlaceSignature={onPlaceSignature}
                   />
 
+                  {/* Inline editor for existing page text (true content editing) */}
+                  {activeTool === 'edit_text' && (
+                    <TextEditLayer
+                      documentId={documentId}
+                      pageIndex={page.index}
+                      width={page.width}
+                      height={page.height}
+                      zoom={zoom}
+                      onEditText={onEditText}
+                    />
+                  )}
+
                   {/* Search highlights overlay */}
                   <SearchOverlayLayer
                     pageIndex={page.index}

frontend/src/viewer/SelectionLayer.tsx

@@ -1,6 +1,9 @@
-import React, { useState, useRef, useMemo } from 'react';
+import React, { useState, useRef, useMemo, useEffect, useCallback } from 'react';
 import type { Point, Rect } from '../lib/coordinateMapping';
 import type { Glyph } from '../lib/gatewayService';
+import { TextSelectionModel, type CaretRange } from '../lib/textSelection';
+
+type Granularity = 'caret' | 'word' | 'line';
 
 interface SelectionLayerProps {
   pageIndex: number;
@@ -8,118 +11,201 @@ interface SelectionLayerProps {
   height: number;
   zoom: number;
   glyphs: Glyph[];
+  /** 'select' keeps the selection live for copy; 'highlight' emits on mouse-up. */
+  mode?: 'select' | 'highlight';
   onTextSelected?: (text: string, bbox: Rect) => void;
 }
 
-interface ZGlyph { gx: number; gy: number; gw: number; gh: number; text: string }
+// Broadcast so that starting a selection on one page clears every other page's.
+const SEL_START_EVT = 'pdf-selection-start';
 
-function rectsIntersect(ax: number, ay: number, aw: number, ah: number, b: Rect) {
-  return ax < b.x + b.width && ax + aw > b.x && ay < b.y + b.height && ay + ah > b.y;
-}
-
-export const SelectionLayer: React.FC<SelectionLayerProps> = ({ width, height, zoom, glyphs, onTextSelected }) => {
-  const [dragStart, setDragStart] = useState<Point | null>(null);
-  const [selectionBox, setSelectionBox] = useState<Rect | null>(null);
+export const SelectionLayer: React.FC<SelectionLayerProps> = ({
+  pageIndex,
+  width,
+  height,
+  zoom,
+  glyphs,
+  mode = 'select',
+  onTextSelected,
+}) => {
   const containerRef = useRef<HTMLDivElement | null>(null);
+  const model = useMemo(() => new TextSelectionModel(glyphs), [glyphs]);
 
-  // Glyphs projected into zoomed pixel space once per glyphs/zoom change.
-  const zGlyphs = useMemo<ZGlyph[]>(
-    () => glyphs.map((g) => ({ gx: g.x * zoom, gy: g.y * zoom, gw: g.w * zoom, gh: g.h * zoom, text: g.text })),
-    [glyphs, zoom],
-  );
+  // Caret selection {start: anchor, end: focus}; unnormalised (start may exceed end).
+  const [sel, setSel] = useState<CaretRange | null>(null);
+  const selRef = useRef<CaretRange | null>(null);
+  selRef.current = sel;
 
-  // Glyphs covered by the current drag box (for live highlight feedback).
-  const hitGlyphs = useMemo<ZGlyph[]>(() => {
-    if (!selectionBox || zGlyphs.length === 0) return [];
-    return zGlyphs.filter((g) => rectsIntersect(g.gx, g.gy, g.gw, g.gh, selectionBox));
-  }, [selectionBox, zGlyphs]);
+  // Active drag (null when idle). `base` is the range fixed at mouse-down.
+  const drag = useRef<{ granularity: Granularity; base: CaretRange } | null>(null);
 
-  const point = (e: React.MouseEvent) => {
-    const rect = containerRef.current!.getBoundingClientRect();
-    return { x: e.clientX - rect.left, y: e.clientY - rect.top };
+  // --- mock-mode fallback (no glyph geometry): raw drag box -------------------
+  const [box, setBox] = useState<Rect | null>(null);
+  const boxStart = useRef<Point | null>(null);
+  const hasGlyphs = model.length > 0;
+
+  const toPage = (e: React.MouseEvent): Point => {
+    const r = containerRef.current!.getBoundingClientRect();
+    return { x: (e.clientX - r.left) / zoom, y: (e.clientY - r.top) / zoom };
   };
 
+  // Merge the mouse-down base range with the range under the cursor, honouring
+  // drag direction (Adobe extends by the original granularity: word/line/caret).
+  const extend = useCallback(
+    (base: CaretRange, granularity: Granularity, p: Point): CaretRange => {
+      if (granularity === 'caret') return { start: base.start, end: model.caretAt(p.x, p.y) };
+      const cur = granularity === 'word' ? model.wordRangeAt(p.x, p.y) : model.lineRangeAt(p.x, p.y);
+      if (!cur) return base;
+      const c = model.caretAt(p.x, p.y);
+      if (c >= base.end) return { start: base.start, end: cur.end };
+      if (c <= base.start) return { start: base.end, end: cur.start };
+      return { start: base.start, end: base.end };
+    },
+    [model],
+  );
+
+  const clear = useCallback(() => {
+    setSel(null);
+    setBox(null);
+    drag.current = null;
+    boxStart.current = null;
+  }, []);
+
+  // Clear this page's selection when another page starts one.
+  useEffect(() => {
+    const onOther = (e: Event) => {
+      if ((e as CustomEvent<number>).detail !== pageIndex) clear();
+    };
+    window.addEventListener(SEL_START_EVT, onOther as EventListener);
+    return () => window.removeEventListener(SEL_START_EVT, onOther as EventListener);
+  }, [pageIndex, clear]);
+
+  // Keyboard: Ctrl/Cmd+C copies, Ctrl/Cmd+A extends to the whole page, Esc clears.
+  useEffect(() => {
+    const onKey = (e: KeyboardEvent) => {
+      const cur = selRef.current;
+      const has = !!cur && cur.start !== cur.end;
+      const mod = e.ctrlKey || e.metaKey;
+      if (mod && e.key.toLowerCase() === 'a' && has) {
+        e.preventDefault();
+        setSel(model.selectAll());
+      } else if (mod && e.key.toLowerCase() === 'c' && has && mode === 'select') {
+        const text = model.textOfRange(cur!);
+        const u = model.unionRect(cur!);
+        if (text && u) onTextSelected?.(text, { x: u.x * zoom, y: u.y * zoom, width: u.w * zoom, height: u.h * zoom });
+      } else if (e.key === 'Escape' && has) {
+        clear();
+      }
+    };
+    window.addEventListener('keydown', onKey);
+    return () => window.removeEventListener('keydown', onKey);
+  }, [model, mode, zoom, onTextSelected, clear]);
+
   const handleMouseDown = (e: React.MouseEvent) => {
-    if (!containerRef.current) return;
-    const p = point(e);
-    setDragStart(p);
-    setSelectionBox({ x: p.x, y: p.y, width: 0, height: 0 });
+    if (!containerRef.current || e.button !== 0) return;
+    e.preventDefault();
+    window.dispatchEvent(new CustomEvent<number>(SEL_START_EVT, { detail: pageIndex }));
+
+    if (!hasGlyphs) {
+      const r = containerRef.current.getBoundingClientRect();
+      const px = { x: e.clientX - r.left, y: e.clientY - r.top };
+      boxStart.current = px;
+      setBox({ x: px.x, y: px.y, width: 0, height: 0 });
+      return;
+    }
+
+    const p = toPage(e);
+    let base: CaretRange;
+    let granularity: Granularity;
+    if (e.shiftKey && selRef.current) {
+      base = { start: selRef.current.start, end: selRef.current.start };
+      granularity = 'caret';
+      setSel({ start: base.start, end: model.caretAt(p.x, p.y) });
+    } else if (e.detail >= 3) {
+      base = model.lineRangeAt(p.x, p.y) ?? { start: model.caretAt(p.x, p.y), end: model.caretAt(p.x, p.y) };
+      granularity = 'line';
+      setSel(base);
+    } else if (e.detail === 2) {
+      base = model.wordRangeAt(p.x, p.y) ?? { start: model.caretAt(p.x, p.y), end: model.caretAt(p.x, p.y) };
+      granularity = 'word';
+      setSel(base);
+    } else {
+      const c = model.caretAt(p.x, p.y);
+      base = { start: c, end: c };
+      granularity = 'caret';
+      setSel(base);
+    }
+    drag.current = { granularity, base };
   };
 
   const handleMouseMove = (e: React.MouseEvent) => {
-    if (!dragStart || !containerRef.current) return;
-    const p = point(e);
-    setSelectionBox({
-      x: Math.min(dragStart.x, p.x),
-      y: Math.min(dragStart.y, p.y),
-      width: Math.abs(dragStart.x - p.x),
-      height: Math.abs(dragStart.y - p.y),
+    if (!containerRef.current) return;
+    if (!hasGlyphs) {
+      if (!boxStart.current) return;
+      const r = containerRef.current.getBoundingClientRect();
+      const px = { x: e.clientX - r.left, y: e.clientY - r.top };
+      setBox({
+        x: Math.min(boxStart.current.x, px.x),
+        y: Math.min(boxStart.current.y, px.y),
+        width: Math.abs(boxStart.current.x - px.x),
+        height: Math.abs(boxStart.current.y - px.y),
       });
-  };
-
-  const buildText = (hits: ZGlyph[]): string => {
-    if (hits.length === 0) return '';
-    // Group into lines by vertical proximity, then order left→right.
-    const sorted = [...hits].sort((a, b) => a.gy - b.gy || a.gx - b.gx);
-    const lines: ZGlyph[][] = [];
-    for (const g of sorted) {
-      const last = lines[lines.length - 1];
-      if (last && Math.abs(last[0].gy - g.gy) < last[0].gh * 0.6) last.push(g);
-      else lines.push([g]);
+      return;
     }
-    return lines
-      .map((line) =>
-        line
-          .sort((a, b) => a.gx - b.gx)
-          .map((g, i, arr) => {
-            const prev = arr[i - 1];
-            const gap = prev ? g.gx - (prev.gx + prev.gw) : 0;
-            const sep = prev && gap > g.gw * 0.4 ? ' ' : '';
-            return sep + g.text;
-          })
-          .join(''),
-      )
-      .join('\n')
-      .replace(/\s+/g, ' ')
-      .trim();
+    if (!drag.current) return;
+    const p = toPage(e);
+    setSel(extend(drag.current.base, drag.current.granularity, p));
   };
 
   const handleMouseUp = () => {
-    if (selectionBox && selectionBox.width > 3 && selectionBox.height > 3) {
-      if (hitGlyphs.length > 0) {
-        // Tight union bbox around actually-selected glyphs (zoomed px).
-        const minX = Math.min(...hitGlyphs.map((g) => g.gx));
-        const minY = Math.min(...hitGlyphs.map((g) => g.gy));
-        const maxX = Math.max(...hitGlyphs.map((g) => g.gx + g.gw));
-        const maxY = Math.max(...hitGlyphs.map((g) => g.gy + g.gh));
-        onTextSelected?.(buildText(hitGlyphs), { x: minX, y: minY, width: maxX - minX, height: maxY - minY });
-      } else {
-        // No glyph data (e.g. mock mode) — fall back to the raw drag box so the
-        // highlight workflow still functions.
-        onTextSelected?.('', selectionBox);
+    // Mock-mode: report the raw drag box so highlighting still works.
+    if (!hasGlyphs) {
+      if (box && box.width > 3 && box.height > 3) onTextSelected?.('', box);
+      setBox(null);
+      boxStart.current = null;
+      return;
     }
+    drag.current = null;
+    const cur = selRef.current;
+    if (!cur || cur.start === cur.end) {
+      setSel(null);
+      return;
     }
-    setDragStart(null);
-    setSelectionBox(null);
+    if (mode === 'highlight') {
+      const text = model.textOfRange(cur);
+      const u = model.unionRect(cur);
+      if (u) onTextSelected?.(text, { x: u.x * zoom, y: u.y * zoom, width: u.w * zoom, height: u.h * zoom });
+      setSel(null);
+    }
+    // 'select' mode: keep the selection live for Ctrl+C.
   };
 
+  const rects = sel ? model.rectsOfRange(sel) : [];
+
   return (
     <div
       ref={containerRef}
-      className="absolute top-0 left-0 z-20 cursor-text"
+      className="absolute top-0 left-0 z-20 cursor-text select-none"
       style={{ width: `${width}px`, height: `${height}px` }}
       onMouseDown={handleMouseDown}
       onMouseMove={handleMouseMove}
       onMouseUp={handleMouseUp}
-      onMouseLeave={handleMouseUp}
+      onMouseLeave={() => {
+        if (drag.current || boxStart.current) handleMouseUp();
+      }}
     >
-      {/* Live per-glyph highlight */}
-      {hitGlyphs.map((g, i) => (
-        <div key={i} className="absolute bg-[rgba(37,99,235,0.28)] pointer-events-none" style={{ left: g.gx, top: g.gy, width: g.gw, height: g.gh }} />
+      {rects.map((q, i) => (
+        <div
+          key={i}
+          className="absolute bg-[rgba(37,99,235,0.30)] pointer-events-none"
+          style={{ left: q.x * zoom, top: q.y * zoom, width: q.w * zoom, height: q.h * zoom }}
+        />
       ))}
-      {/* Drag rectangle (only when nothing is being hit, e.g. mock mode) */}
-      {selectionBox && hitGlyphs.length === 0 && (
-        <div className="absolute bg-[rgba(37,99,235,0.22)] pointer-events-none rounded-[1px]" style={{ left: selectionBox.x, top: selectionBox.y, width: selectionBox.width, height: selectionBox.height }} />
+      {box && (
+        <div
+          className="absolute bg-[rgba(37,99,235,0.22)] pointer-events-none rounded-[1px]"
+          style={{ left: box.x, top: box.y, width: box.width, height: box.height }}
+        />
       )}
     </div>
   );

frontend/src/viewer/TextEditLayer.tsx

@@ -0,0 +1,152 @@
+import React, { useEffect, useMemo, useRef, useState } from 'react';
+import { gatewayService } from '../lib/gatewayService';
+
+// A single editable run, in PDF BOTTOM-LEFT page space (as getPageModel returns).
+export interface EditableRun {
+  text: string;
+  x: number;
+  y: number;
+  w: number;
+  h: number;
+  fontSize: number;
+}
+
+interface TextEditLayerProps {
+  documentId: string;
+  pageIndex: number;
+  width: number; // page width in ZOOMED px (= widthPts * zoom)
+  height: number; // page height in ZOOMED px (= heightPts * zoom)
+  zoom: number;
+  onEditText?: (pageIndex: number, run: EditableRun, newText: string) => void;
+}
+
+// Flatten the dynamic page-model JSON into a flat list of editable runs.
+function flattenRuns(model: any): EditableRun[] {
+  const runs: EditableRun[] = [];
+  const paragraphs = model?.paragraphs ?? [];
+  for (const p of paragraphs) {
+    for (const line of p.lines ?? []) {
+      for (const r of line.runs ?? []) {
+        if (typeof r.text === 'string' && r.text.trim() && r.w > 0 && r.h > 0) {
+          runs.push({ text: r.text, x: r.x, y: r.y, w: r.w, h: r.h, fontSize: r.font_size ?? r.h });
+        }
+      }
+    }
+  }
+  return runs;
+}
+
+export const TextEditLayer: React.FC<TextEditLayerProps> = ({
+  documentId,
+  pageIndex,
+  width,
+  height,
+  zoom,
+  onEditText,
+}) => {
+  const [runs, setRuns] = useState<EditableRun[]>([]);
+  const [editing, setEditing] = useState<number | null>(null);
+  const [value, setValue] = useState('');
+  const committedRef = useRef(false);
+
+  // Fetch the page model once per document/page.
+  useEffect(() => {
+    let cancelled = false;
+    setEditing(null);
+    gatewayService
+      .getPageModel(documentId, pageIndex)
+      .then((model) => {
+        if (!cancelled) setRuns(flattenRuns(model));
+      })
+      .catch(() => {
+        if (!cancelled) setRuns([]);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [documentId, pageIndex]);
+
+  // Page height in points (props are zoomed px) — needed for the bottom-left→top-left flip.
+  const heightPts = height / zoom;
+
+  // Display rect (top-left, zoomed px) for a run whose coords are bottom-left points.
+  const rectOf = (r: EditableRun) => ({
+    left: r.x * zoom,
+    top: (heightPts - (r.y + r.h)) * zoom,
+    width: r.w * zoom,
+    height: r.h * zoom,
+  });
+
+  const openEditor = (i: number) => {
+    committedRef.current = false;
+    setEditing(i);
+    setValue(runs[i].text);
+  };
+
+  const commit = () => {
+    if (editing === null || committedRef.current) return;
+    committedRef.current = true;
+    const run = runs[editing];
+    const next = value;
+    setEditing(null);
+    if (next !== run.text) onEditText?.(pageIndex, run, next);
+  };
+
+  const cancel = () => {
+    committedRef.current = true;
+    setEditing(null);
+  };
+
+  const editingRect = useMemo(
+    () => (editing !== null && runs[editing] ? rectOf(runs[editing]) : null),
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+    [editing, runs, zoom, height],
+  );
+
+  return (
+    <div className="absolute top-0 left-0 z-[35]" style={{ width: `${width}px`, height: `${height}px` }}>
+      {/* Per-run hit targets (visible hint on hover). */}
+      {editing === null &&
+        runs.map((r, i) => {
+          const box = rectOf(r);
+          return (
+            <div
+              key={i}
+              title="Click to edit this text"
+              onClick={() => openEditor(i)}
+              className="absolute cursor-text rounded-[2px] hover:bg-[rgba(37,99,235,0.12)] hover:outline hover:outline-1 hover:outline-[#2563eb]"
+              style={{ left: box.left, top: box.top, width: box.width, height: box.height }}
+            />
+          );
+        })}
+
+      {/* Inline editor for the selected run. */}
+      {editing !== null && editingRect && (
+        <textarea
+          autoFocus
+          value={value}
+          onChange={(e) => setValue(e.target.value)}
+          onBlur={commit}
+          onKeyDown={(e) => {
+            if (e.key === 'Enter' && !e.shiftKey) {
+              e.preventDefault();
+              commit();
+            }
+            if (e.key === 'Escape') {
+              e.preventDefault();
+              cancel();
+            }
+          }}
+          className="absolute z-[36] bg-white border-[1.5px] border-[#2563eb] rounded-[3px] shadow-[0_4px_12px_rgba(16,24,40,0.12)] outline-none resize-none overflow-hidden font-sans leading-[1.1] px-[3px] py-[1px] whitespace-nowrap"
+          style={{
+            left: editingRect.left,
+            top: editingRect.top,
+            minWidth: editingRect.width,
+            height: Math.max(editingRect.height, (runs[editing].fontSize + 4) * zoom),
+            fontSize: `${runs[editing].fontSize * zoom}px`,
+          }}
+        />
+      )}
+    </div>
+  );
+};

gateway/app/routers/edits.py

@@ -168,8 +168,29 @@ class PageReorderOperation(BaseModel):
     data: PageReorderData
 
 
+class EditTextData(BaseModel):
+    # Target line/run bbox in PDF bottom-left page space (frontend does the Y-flip).
+    x: float
+    y: float
+    width: float = Field(..., gt=0)
+    height: float = Field(..., gt=0)
+    newText: str
+    originalText: str | None = None
+    fontSize: float | None = Field(default=None, gt=0)
+    color: str | None = None
+    fallbackFont: str = "Helvetica"
+
+
+class EditTextOperation(BaseModel):
+    id: str
+    type: Literal["edit_text"]
+    pageIndex: int = Field(..., ge=0)
+    data: EditTextData
+
+
 class UpdateFieldData(BaseModel):
     value: str | bool
+    annotationId: str | None = None
 
 
 class UpdateFieldOperation(BaseModel):
@@ -221,7 +242,8 @@ EditOperation = Annotated[
     | PageReorderOperation
     | UpdateFieldOperation
     | DeleteAnnotationOperation
-    | UpdateAnnotationOperation,
+    | UpdateAnnotationOperation
+    | EditTextOperation,
     Field(discriminator="type"),
 ]
 
@@ -295,8 +317,11 @@ def apply_edits_impl(document_id: str, request: EditsRequest):
         edits_json = json.dumps(req_dict)
         doc.apply_edits(edits_json)
 
-        has_redaction = any(op.get("type") == "redaction" for op in req_dict.get("operations", []))
-        new_bytes = doc.save_full() if has_redaction else doc.save_incremental()
+        # Redaction and in-place text rewrites mutate existing objects, which do
+        # not round-trip cleanly through an incremental save — force a full save.
+        full_save_types = {"redaction", "edit_text"}
+        needs_full = any(op.get("type") in full_save_types for op in req_dict.get("operations", []))
+        new_bytes = doc.save_full() if needs_full else doc.save_incremental()
 
         new_doc = pdfengine.PdfDocument.load_from_memory(new_bytes)
 

gateway/app/routers/render.py

@@ -56,7 +56,14 @@ def extract_page_text(document_id: str, page_index: Annotated[int, Path(ge=0)]):
         annots = page.extract_annotations_text()
         if annots:
             text += "\n" + "\n".join(annots)
+        # The engine emits glyph bounds in PDFium-native BOTTOM-LEFT space (y = bbox
+        # bottom). The frontend renders pages top-down, so flip to TOP-LEFT here —
+        # the same convention /search and the annotation/highlight paths already use.
+        # Without this, text selection is vertically mirrored.
+        page_height = page.height
         glyphs = page.extract_text_with_bounds()
+        for g in glyphs:
+            g["y"] = page_height - (g["y"] + g["h"])
         return {"text": text, "glyphs": glyphs}
     except IndexError:
         raise HTTPException(

scripts/fetch_corpus.py

@@ -0,0 +1,130 @@
+#!/usr/bin/env python3
+"""Download a large PDF corpus for fuzzing into a gitignored directory.
+
+The committed `corpus/` holds a small, curated set of fixtures used by the
+rendering-regression baseline. Fuzzing wants *volume and variety* instead, so we
+pull hundreds of real-world PDFs from public test suites into `corpus/fuzz/`,
+which is gitignored.
+
+    python scripts/fetch_corpus.py                 # default: ~600 from pdf.js
+    python scripts/fetch_corpus.py --limit 200
+    python scripts/fetch_corpus.py --source pdfium # GoogleTest pdfium corpus
+
+Network failures are tolerated: whatever downloads is usable, and re-running only
+fetches what's missing. Only standard-library modules are used.
+"""
+
+from __future__ import annotations
+
+import argparse
+import hashlib
+import json
+import sys
+import urllib.error
+import urllib.request
+from pathlib import Path
+
+# Stable raw-file base for the pinned manifest (filename → bytes).
+_RAW_BASE = "https://raw.githubusercontent.com/mozilla/pdf.js/master/test/pdfs"
+
+ROOT = Path(__file__).resolve().parents[1]
+DEST = ROOT / "corpus" / "fuzz"
+
+# GitHub "contents" API listings of directories full of .pdf files.
+SOURCES = {
+    "pdfjs": "https://api.github.com/repos/mozilla/pdf.js/contents/test/pdfs?ref=master",
+    "pdfium": "https://api.github.com/repos/PDFium/pdfium/contents/testing/resources?ref=main",
+}
+
+_HEADERS = {"User-Agent": "pdfengine-fetch-corpus", "Accept": "application/vnd.github+json"}
+
+
+def _get(url: str, raw: bool = False) -> bytes:
+    req = urllib.request.Request(url, headers=_HEADERS)
+    with urllib.request.urlopen(req, timeout=30) as resp:  # noqa: S310 (trusted hosts)
+        return resp.read()
+
+
+def _fetch_from_manifest(manifest_path: Path, dest: Path) -> int:
+    """Download exactly the files in the pinned manifest and verify their SHA-256,
+    so every developer and CI run gets a byte-identical corpus."""
+    manifest = json.loads(manifest_path.read_text())
+    files = manifest.get("files", [])
+    print(f"Manifest: {len(files)} pinned files -> {dest} (verifying SHA-256)")
+    got = existed = failed = mismatch = 0
+    for entry in files:
+        name, want = entry["name"], entry["sha256"]
+        out = dest / name
+        if out.exists() and hashlib.sha256(out.read_bytes()).hexdigest() == want:
+            existed += 1
+            continue
+        try:
+            data = _get(f"{_RAW_BASE}/{name}", raw=True)
+        except (urllib.error.URLError, OSError) as exc:
+            print(f"  fail {name} ({exc})")
+            failed += 1
+            continue
+        have = hashlib.sha256(data).hexdigest()
+        if have != want:
+            print(f"  MISMATCH {name}: expected {want[:12]}..., got {have[:12]}... (skipped)")
+            mismatch += 1
+            continue
+        out.write_bytes(data)
+        got += 1
+        if got % 50 == 0:
+            print(f"  ... {got} verified")
+    total = len(list(dest.glob("*.pdf")))
+    print(f"\nDone. +{got} new, {existed} already present & verified, "
+          f"{failed} failed, {mismatch} hash-mismatch.")
+    print(f"Corpus now holds {total} PDFs at {dest}")
+    return 0 if mismatch == 0 else 1
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description=__doc__)
+    ap.add_argument("--source", choices=sorted(SOURCES), default="pdfjs")
+    ap.add_argument("--limit", type=int, default=600, help="max files to download")
+    ap.add_argument("--dest", type=Path, default=DEST)
+    ap.add_argument("--manifest", type=Path, default=None,
+                    help="reproduce the exact pinned corpus from a manifest (verifies SHA-256)")
+    args = ap.parse_args()
+
+    args.dest.mkdir(parents=True, exist_ok=True)
+
+    # Manifest mode: deterministic, hash-verified, identical for everyone.
+    if args.manifest:
+        return _fetch_from_manifest(args.manifest, args.dest)
+
+    print(f"Listing {args.source} corpus ...")
+    try:
+        listing = json.loads(_get(SOURCES[args.source]))
+    except (urllib.error.URLError, json.JSONDecodeError) as exc:
+        print(f"ERROR: could not list corpus ({exc}). Check your network / GitHub rate limit.")
+        return 2
+
+    pdfs = [e for e in listing if e.get("name", "").lower().endswith(".pdf") and e.get("download_url")]
+    print(f"Found {len(pdfs)} PDFs; downloading up to {args.limit} into {args.dest} ...")
+
+    got = failed = existed = 0
+    for entry in pdfs[: args.limit]:
+        out = args.dest / entry["name"]
+        if out.exists() and out.stat().st_size > 0:
+            existed += 1
+            continue
+        try:
+            out.write_bytes(_get(entry["download_url"], raw=True))
+            got += 1
+            if got % 25 == 0:
+                print(f"  ... {got} downloaded")
+        except (urllib.error.URLError, OSError) as exc:
+            print(f"  fail {entry['name']} ({exc})")
+            failed += 1
+
+    total = len(list(args.dest.glob("*.pdf")))
+    print(f"\nDone. +{got} new, {existed} already present, {failed} failed.")
+    print(f"Corpus now holds {total} PDFs at {args.dest}")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())

tests/edits/test_edit_text.py

@@ -0,0 +1,130 @@
+#!/usr/bin/env python3
+"""Round-trip tests for the `edit_text` op (line-level text rewriting).
+
+Verifies that editing an existing text object: (1) actually changes the rendered
+text, (2) keeps the result within the original line's horizontal bounds (the
+acceptance criterion), and (3) fails closed when the target bbox matches nothing.
+
+Run directly:  gateway/.venv/Scripts/python.exe tests/edits/test_edit_text.py
+"""
+
+from __future__ import annotations
+
+import json
+import sys
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+sys.path.insert(0, str(ROOT / "gateway"))
+import pdfengine  # noqa: E402
+
+CORPUS = ROOT / "corpus" / "fonts" / "utf-8.pdf"
+EPS = 2.0  # points; tolerance for rounding / glyph metrics
+
+
+def _load():
+    return pdfengine.PdfDocument.load_from_memory(CORPUS.read_bytes(), "")
+
+
+def _runs(model):
+    return [r for p in model.paragraphs for l in p.lines for r in l.runs]
+
+
+def _first_run(model):
+    return model.paragraphs[0].lines[0].runs[0]
+
+
+def _edit_op(model, run, new_text: str):
+    # Engine model coords are PDFium-native BOTTOM-LEFT (y = bbox bottom), the same
+    # space the op + FPDFPageObj_GetBounds use — so no flip here.
+    return {
+        "version": "1.0",
+        "operations": [{
+            "id": "e1", "type": "edit_text", "pageIndex": 0,
+            "data": {
+                "x": run.x,
+                "y": run.y,
+                "width": run.w,
+                "height": run.h,
+                "newText": new_text,
+                "originalText": run.text,
+                "fontSize": run.font_size,
+                "fallbackFont": "Helvetica",
+            },
+        }],
+    }
+
+
+def test_roundtrip_replaces_text():
+    doc = _load()
+    model = doc.get_page(0).extract_document_model()
+    run = _first_run(model)
+    orig_x, orig_w, orig_y = run.x, run.w, run.y
+    doc.apply_edits(json.dumps(_edit_op(model, run, "REPLACED")))
+    out = doc.save_full()
+
+    m2 = pdfengine.PdfDocument.load_from_memory(out, "").get_page(0).extract_document_model()
+    edited = next((r for r in _runs(m2) if "REPLACED" in r.text), None)
+    assert edited is not None, "edited text 'REPLACED' not found after round-trip"
+    # stays within the original horizontal bounds
+    assert edited.x >= orig_x - EPS, f"left moved out of bounds: {edited.x} < {orig_x}"
+    assert edited.x + edited.w <= orig_x + orig_w + EPS, \
+        f"right overflowed bounds: {edited.x + edited.w} > {orig_x + orig_w}"
+    # baseline / vertical position preserved
+    assert abs(edited.y - orig_y) < EPS + run.h, f"baseline drifted: {edited.y} vs {orig_y}"
+    print(f"  ok  roundtrip: '{run.text[:20]}' -> '{edited.text[:20]}' "
+          f"(x {orig_x:.0f}..{orig_x + orig_w:.0f} -> {edited.x:.0f}..{edited.x + edited.w:.0f})")
+
+
+def test_long_text_squeezes_within_bounds():
+    doc = _load()
+    model = doc.get_page(0).extract_document_model()
+    run = _first_run(model)
+    orig_right = run.x + run.w
+    long_text = "This is a deliberately very long replacement string to force squeeze"
+    doc.apply_edits(json.dumps(_edit_op(model, run, long_text)))
+    out = doc.save_full()
+
+    m2 = pdfengine.PdfDocument.load_from_memory(out, "").get_page(0).extract_document_model()
+    edited = next((r for r in _runs(m2) if "deliberately" in r.text), None)
+    assert edited is not None, "long edited text not found"
+    assert edited.x + edited.w <= orig_right + EPS, \
+        f"long text overflowed: {edited.x + edited.w:.1f} > {orig_right:.1f}"
+    print(f"  ok  squeeze: {len(long_text)} chars fit in {run.w:.0f}pt "
+          f"(right {edited.x + edited.w:.0f} <= {orig_right:.0f})")
+
+
+def test_no_match_fails():
+    doc = _load()
+    model = doc.get_page(0).extract_document_model()
+    op = {
+        "version": "1.0",
+        "operations": [{
+            "id": "e1", "type": "edit_text", "pageIndex": 0,
+            "data": {"x": 5000.0, "y": 5000.0, "width": 50.0, "height": 10.0,
+                     "newText": "X", "fallbackFont": "Helvetica"},
+        }],
+    }
+    try:
+        doc.apply_edits(json.dumps(op))
+    except Exception:
+        print("  ok  no-match correctly raised")
+        return
+    raise AssertionError("edit_text with an off-page bbox should have failed, but succeeded")
+
+
+def main() -> int:
+    tests = [test_roundtrip_replaces_text, test_long_text_squeezes_within_bounds, test_no_match_fails]
+    failed = 0
+    for t in tests:
+        try:
+            t()
+        except AssertionError as exc:
+            print(f"  FAIL {t.__name__}: {exc}")
+            failed += 1
+    print(f"\n{len(tests) - failed}/{len(tests)} passed.")
+    return 1 if failed else 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

tests/regression/README.md

@@ -0,0 +1,72 @@
+# Rendering-regression harness
+
+Renders the committed `corpus/` with the PDFium engine and compares each page to
+a **frozen baseline** using SSIM. A failure means *this* build renders
+differently from the blessed build — not that it disagrees with Acrobat.
+
+## Prerequisites
+
+- The engine is built and copied to `gateway/` (run `scripts/build_cpp.ps1 -Preset win-local-pdfium`).
+- The gateway venv has `numpy` and `Pillow` (`gateway/.venv`).
+
+## Usage
+
+```bash
+# Check the current build against the frozen baseline (CI mode; non-zero exit on regression)
+gateway/.venv/Scripts/python.exe tests/regression/run.py
+
+# Re-freeze the baseline after an *intentional* rendering change (review the diff!)
+gateway/.venv/Scripts/python.exe tests/regression/run.py --update
+```
+
+Options: `--dpi 72`, `--max-pages 2`, `--threshold 0.990`, `--corpus`, `--baseline`.
+
+## Large corpus (500+ real-world PDFs)
+
+The corpus is **pinned and shared via a manifest** so every developer and CI run
+gets a byte-identical set. The 600 PDFs themselves are gitignored (≈68 MB); only
+the manifest (`corpus-manifest.json`, name + SHA-256 per file, ≈96 KB) is
+committed. Reproduce the exact corpus with:
+
+```bash
+python scripts/fetch_corpus.py --manifest tests/regression/corpus-manifest.json
+```
+
+This downloads each pinned file and **verifies its SHA-256** — a mismatch is
+reported and skipped, so the corpus can never silently drift between machines.
+(`scripts/fetch_corpus.py` with no `--manifest` just grabs "latest" from pdf.js,
+which is *not* reproducible — use it only to refresh/regenerate the manifest.)
+
+Two ways to use the corpus for regression:
+
+```bash
+# 1. Render-stability sweep — no baseline; passes as long as the engine never
+#    crashes on any real-world PDF. The CI-friendly form of a 500+ regression.
+python scripts/fetch_corpus.py
+gateway/.venv/Scripts/python.exe tests/regression/run.py --sweep --corpus corpus/fuzz --max-pages 3
+
+# 2. SSIM baseline over the large corpus (baseline is gitignored — it is derived
+#    from gitignored inputs and is ~tens of MB, so it lives locally / as a CI
+#    artifact, not in git).
+... run.py --update --corpus corpus/fuzz --baseline tests/regression/baseline-large
+... run.py          --corpus corpus/fuzz --baseline tests/regression/baseline-large
+```
+
+The committed `baseline/` stays the small curated set (reviewable in PRs);
+`baseline-large/` is the throwaway large-corpus reference. In CI the libFuzzer
+binary doubles as a sweep via `pdfengine_fuzz -runs=0 corpus/fuzz/` (see
+`.github/workflows/fuzz.yml`).
+
+## How it works
+
+- `ssim.py` — dependency-light SSIM (numpy only) via an integral-image box filter.
+  Discriminates correctly: identical→1.0, shifted→~0.81, noise→~0.00, shape
+  mismatch→0.0.
+- `run.py` — enumerates `corpus/**/*.pdf` (excluding the gitignored `corpus/fuzz/`),
+  renders to grayscale, and scores against `baseline/*.png`.
+- `baseline/` — committed PNGs. Because the baseline is the engine's *own* prior
+  output, it is tied to this PDFium version; re-freeze deliberately when PDFium
+  is upgraded.
+
+Encrypted / intentionally-malformed fixtures are skipped (and reported), not
+failed.

tests/regression/run.py

@@ -0,0 +1,139 @@
+#!/usr/bin/env python3
+"""Rendering-regression harness for the PDFium engine.
+
+Renders every page (capped) of every PDF in the committed corpus and compares it
+to a frozen baseline using SSIM. The baseline is the engine's own output at the
+moment it was approved — so a regression here means *this* build renders
+differently from the blessed build, not that it disagrees with Acrobat.
+
+    python tests/regression/run.py --update     # (re)generate the frozen baseline
+    python tests/regression/run.py              # check current renders vs baseline
+
+Exit code is non-zero if any page regresses below the SSIM threshold, so it
+drops straight into CI.
+"""
+
+from __future__ import annotations
+
+import argparse
+import io
+import sys
+from pathlib import Path
+
+import numpy as np
+from PIL import Image
+
+sys.path.insert(0, str(Path(__file__).resolve().parent))
+from ssim import ssim  # noqa: E402
+
+ROOT = Path(__file__).resolve().parents[2]
+DEFAULT_CORPUS = ROOT / "corpus"
+DEFAULT_BASELINE = Path(__file__).resolve().parent / "baseline"
+# corpus subdirs that are *not* part of the frozen baseline (large/downloaded).
+EXCLUDE_DIRS = {"fuzz"}
+
+
+def _import_engine():
+    # The compiled extension lives in gateway/ after scripts/build_cpp.ps1.
+    sys.path.insert(0, str(ROOT / "gateway"))
+    import pdfengine  # noqa: PLC0415
+
+    return pdfengine
+
+
+def _render_gray(page, dpi: int) -> np.ndarray:
+    png = page.render(dpi).data
+    img = Image.open(io.BytesIO(png)).convert("L")
+    return np.asarray(img)
+
+
+def _corpus_pdfs(corpus: Path):
+    for p in sorted(corpus.rglob("*.pdf")):
+        if any(part in EXCLUDE_DIRS for part in p.relative_to(corpus).parts):
+            continue
+        yield p
+
+
+def _key(rel: Path, page_index: int) -> str:
+    return f"{rel.as_posix().replace('/', '__')}__p{page_index}"
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description=__doc__)
+    ap.add_argument("--update", action="store_true", help="write the frozen baseline instead of checking")
+    ap.add_argument("--sweep", action="store_true",
+                    help="render-stability sweep over a large corpus (no baseline); "
+                         "passes as long as the engine never crashes")
+    ap.add_argument("--corpus", type=Path, default=DEFAULT_CORPUS)
+    ap.add_argument("--baseline", type=Path, default=DEFAULT_BASELINE)
+    ap.add_argument("--dpi", type=int, default=72)
+    ap.add_argument("--max-pages", type=int, default=2, help="pages rendered per document")
+    ap.add_argument("--threshold", type=float, default=0.990, help="min SSIM to pass")
+    args = ap.parse_args()
+
+    pdfengine = _import_engine()
+    args.baseline.mkdir(parents=True, exist_ok=True)
+
+    failures: list[tuple[str, float]] = []
+    checked = skipped = updated = rendered = 0
+
+    for pdf in _corpus_pdfs(args.corpus):
+        rel = pdf.relative_to(args.corpus)
+        try:
+            doc = pdfengine.PdfDocument.load_from_memory(pdf.read_bytes(), "")
+            n = min(doc.page_count, args.max_pages)
+        except Exception as exc:  # encrypted / intentionally-malformed fixtures
+            print(f"  skip  {rel}  ({exc})")
+            skipped += 1
+            continue
+
+        for i in range(n):
+            try:
+                cur = _render_gray(doc.get_page(i), args.dpi)
+            except Exception as exc:
+                print(f"  skip  {rel} p{i}  (render: {exc})")
+                skipped += 1
+                continue
+
+            if args.sweep:
+                # Surviving the render is the whole test; a crash kills the process.
+                rendered += 1
+                continue
+
+            ref_path = args.baseline / f"{_key(rel, i)}.png"
+            if args.update:
+                Image.fromarray(cur).save(ref_path)
+                updated += 1
+                continue
+
+            if not ref_path.exists():
+                print(f"  NEW   {rel} p{i}  (no baseline — run --update)")
+                failures.append((f"{rel} p{i}", -1.0))
+                continue
+
+            ref = np.asarray(Image.open(ref_path).convert("L"))
+            score = ssim(cur, ref)
+            checked += 1
+            mark = "ok   " if score >= args.threshold else "FAIL "
+            if score < args.threshold:
+                failures.append((f"{rel} p{i}", score))
+            print(f"  {mark} {rel} p{i}  SSIM={score:.4f}")
+
+    print()
+    if args.sweep:
+        print(f"Sweep complete: {rendered} pages rendered, {skipped} skipped (graceful). "
+              f"No crash — engine is render-stable over this corpus.")
+        return 0
+    if args.update:
+        print(f"Baseline updated: {updated} images written to {args.baseline}")
+        return 0
+
+    print(f"Checked {checked} pages, skipped {skipped}, {len(failures)} regression(s).")
+    for name, score in failures:
+        tag = "missing baseline" if score < 0 else f"SSIM={score:.4f}"
+        print(f"  REGRESSION: {name}  ({tag})")
+    return 1 if failures else 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

tests/regression/ssim.py

@@ -0,0 +1,57 @@
+"""Windowed SSIM with no scipy/skimage dependency.
+
+Implements the Wang et al. structural-similarity index using a uniform window
+(box filter) computed via an integral image — fast, vectorised, and dependency
+-light (numpy only). Good enough to catch rendering regressions while tolerating
+sub-pixel antialiasing noise.
+"""
+
+from __future__ import annotations
+
+import numpy as np
+
+_C1 = (0.01 * 255) ** 2
+_C2 = (0.03 * 255) ** 2
+
+
+def _mean_filter(img: np.ndarray, k: int) -> np.ndarray:
+    """Box-mean over a k×k window, same shape, edge-clamped window area."""
+    h, w = img.shape
+    pad = k // 2
+    integral = np.zeros((h + 1, w + 1), dtype=np.float64)
+    integral[1:, 1:] = img.cumsum(0).cumsum(1)
+
+    ys = np.clip(np.arange(h) - pad, 0, h)
+    ye = np.clip(np.arange(h) + pad + 1, 0, h)
+    xs = np.clip(np.arange(w) - pad, 0, w)
+    xe = np.clip(np.arange(w) + pad + 1, 0, w)
+
+    a = integral[ye][:, xe]
+    b = integral[ye][:, xs]
+    c = integral[ys][:, xe]
+    d = integral[ys][:, xs]
+    total = a - b - c + d
+    area = (ye - ys)[:, None] * (xe - xs)[None, :]
+    return total / area
+
+
+def ssim(a: np.ndarray, b: np.ndarray, k: int = 7) -> float:
+    """Mean SSIM in [0, 1]. Mismatched shapes score 0.0 (always a regression)."""
+    if a.shape != b.shape:
+        return 0.0
+    a = a.astype(np.float64)
+    b = b.astype(np.float64)
+
+    mu_a = _mean_filter(a, k)
+    mu_b = _mean_filter(b, k)
+    mu_a2 = mu_a * mu_a
+    mu_b2 = mu_b * mu_b
+    mu_ab = mu_a * mu_b
+
+    var_a = _mean_filter(a * a, k) - mu_a2
+    var_b = _mean_filter(b * b, k) - mu_b2
+    cov_ab = _mean_filter(a * b, k) - mu_ab
+
+    num = (2 * mu_ab + _C1) * (2 * cov_ab + _C2)
+    den = (mu_a2 + mu_b2 + _C1) * (var_a + var_b + _C2)
+    return float(np.clip(num / den, 0.0, 1.0).mean())