Draft status

Diagnostic-only draft for #188724.

This is not a WebKit default enablement PR. It does not flip webkit: true,
and it does not claim the Mobile Safari Skwasm canvas sizing issue is fixed.

What changed

Adds compile-time gated RenderCanvas sizing diagnostics behind:

FLUTTER_WEB_DEBUG_RENDER_CANVAS_SIZING

When enabled, the logs capture the values needed to separate the Mobile Safari
failure branches:

• requested bitmap size
• _pixelWidth / _pixelHeight
• canvas width/height attributes
• EngineFlutterDisplay.instance.devicePixelRatio
• window.devicePixelRatio
• visualViewport width/height/scale
• documentElement.clientWidth/clientHeight
• inline and computed canvas CSS width/height
• canvas and host bounding rects
• the same state on the next animation frame after logical-size writes and
  bitmap transfer

Why

On physical iPhone Mobile Safari, a Skwasm build can load skwasm_heavy,
main.dart.mjs, and main.dart.wasm, complete benchmark profiles, and produce
non-empty canvas pixels while the visible canvas CSS rect collapses to about
0.328125 x 0.328125px.

The same failing evidence class has a 1206 x 2142 backing store and a browser
viewport of about 402 x 714 at DPR 3. A local-only forced CSS probe that
sets canvas.style.width = canvas.width / devicePixelRatio and
canvas.style.height = canvas.height / devicePixelRatio recovers the visible
surface. The next step is to prove whether the bad value is a stale pixel size,
DPR/viewport timing, or a post-write CSS overwrite.

Refs issue #188723.

Validation

• ./bin/dart format engine/src/flutter/lib/web_ui/lib/src/engine/compositing/render_canvas.dart
• Targeted RenderCanvas invariant test with GCS artifacts:

DART_SDK_DIR=/Users/marlonjd/Developer/flutter/bin/cache/dart-sdk \
  engine/src/flutter/lib/web_ui/dev/felt test \
  --gcs-prod \
  --browser chrome \
  --compiler dart2js \
  --renderer canvaskit \
  --fail-early \
  test/engine/compositing/render_canvas_test.dart

• Result: passed locally with --gcs-prod; chrome-dart2js-canvaskit-engine,
  3/3 tests.
• Local SDK build setup:

PATH=/Users/marlonjd/Developer/demo/skwasm-pr-188628/.tooling/depot_tools:$PATH \
DART_SDK_DIR=/Users/marlonjd/Developer/demo/skwasm-pr-188628/flutter-pr-worktree/bin/cache/dart-sdk \
PUB_CACHE=/Users/marlonjd/Developer/demo/skwasm-pr-188628/.pub-cache \
  engine/src/flutter/lib/web_ui/dev/felt build sdk canvaskit skwasm skwasm_heavy

• Result: passed locally for SDK/artifact production; local outputs include
  flutter_web_sdk/flutter_js/flutter.js and
  canvaskit/{canvaskit,skwasm,skwasm_heavy}.{js,wasm}.

Diagnostic runtime follow-up

The first local rebuilt SDK diagnostic/no-log Simulator bundles were blocked
before canvas creation with:

RuntimeError: Unreachable code should not be executed

from canvaskit/skwasm_heavy.js. Those runs had profiles: [],
renderCanvasSizingLogs=[], and did not reach the intended canvas sizing logs.

Controls narrowed that blocker:

• safe/no-log local rebuilt SDK controls still hit the same runtime error.
• swapping only cached skwasm_heavy.wasm into the local rebuilt bundle did
  not recover the run.
• full cached-SDK control completed all three profiles with errors: [] and
  normal 402 x 714 canvas geometry on the same Simulator.
• cached full canvaskit/ did not rescue local main.dart.wasm /
  main.dart.mjs.
• cached main output completed with the local rebuilt full canvaskit/
  directory.
• removing only the RenderCanvas diagnostic source patch changed app wasm
  contents but left the local main.dart.mjs support-runtime shape unchanged.
• the local --local-web-sdk path invoked engine-prebuilt Dart 3.12.1 with
  kernel format 130, while the passing cached-SDK path used Dart
  3.13.0-254.0.dev with kernel format 136.

After aligning the local engine-prebuilt Dart SDK symlinks to the cached
3.13.0-254.0.dev Dart SDK and rebuilding the local platform dill set, the
aligned local diagnostic bundle moved to the cached-like dart2wasm support
runtime shape and reached RenderCanvas logs.

Aligned diagnostic bundle:

diagnostic_bundles/20260630T_aligned_dart_sdk_local_web_sdk_diagnostic/bundle

Simulator MobileSafari one-benchmark run:

iphone_evidence_runs/20260630T020038_simulator_skwasm_aligned_dart_sdk_diagnostic_one_benchmark.json

• Result: all-benchmarks-complete
• Loaded artifacts: skwasm_heavy, main.dart.mjs, main.dart.wasm
• Profiles: one bench_card_infinite_scroll profile
• Errors: []
• RenderCanvas sizing log count: 1813
• Representative log after sizing:
  requested=1206x2142, pixel=1206x2142, canvasAttr=1206.0x2142.0,
  [email protected], inlineStyle=402px/714px,
  computedStyle=402px/714px, [email protected],0.0

Physical iPhone MobileSafari one-benchmark run:

iphone_evidence_runs/20260630T020352_physical_iphone_skwasm_aligned_dart_sdk_diagnostic_one_benchmark.json

• Result: all-benchmarks-complete
• Loaded artifacts: skwasm_heavy, main.dart.mjs, main.dart.wasm
• Profiles: one bench_card_infinite_scroll profile
• Errors: []
• RenderCanvas sizing log count: 1807
• Representative log after sizing:
  requested=1206x2142, pixel=1206x2142, canvasAttr=1206.0x2142.0,
  [email protected], inlineStyle=402px/714px,
  computedStyle=402px/714px, [email protected],0.0
• Visual note: the local operator reported that the full scroll content was
  visible during the physical run. An automated physical screenshot is still
  separate evidence and was not captured in this one-benchmark run.

Physical iPhone manual-screenshot follow-up:

iphone_evidence_runs/20260630T021605_physical_iphone_skwasm_aligned_dart_sdk_diagnostic_manual_screenshot_window.json

• Result: all-benchmarks-complete
• Loaded artifacts: skwasm_heavy, main.dart.mjs, main.dart.wasm
• Profiles: all three macrobenchmarks completed
• Errors: []
• RenderCanvas sizing log count: 5421
• Representative log after sizing:
  requested=1206x2142, pixel=1206x2142, canvasAttr=1206.0x2142.0,
  [email protected], inlineStyle=402px/714px,
  computedStyle=402px/714px, [email protected],0.0
• Manual screenshots captured from physical iPhone MobileSafari show visible
  benchmark content for the card, lazy text, and wrapbox scroll benchmarks:
  20260630T021605_physical_iphone_skwasm_aligned_dart_sdk_diagnostic_manual_screenshot_window_card_scroll.png,
  20260630T021605_physical_iphone_skwasm_aligned_dart_sdk_diagnostic_manual_screenshot_window_text_scroll.png,
  20260630T021605_physical_iphone_skwasm_aligned_dart_sdk_diagnostic_manual_screenshot_window_wrapbox_scroll.png.

RenderCanvas log-vs-collapse analysis:

diagnostic_bundles/20260630T_render_canvas_log_vs_collapse_analysis/render_canvas_log_vs_collapse_analysis.md

• Collapsed physical/simulator snapshots had valid 1206 x 2142 backing store
  and non-empty pixels, but inline/computed CSS and canvas rect collapsed to
  0.328125px / 0.328125px.
• Aligned diagnostic logs show _ensureSize setting 1206 x 2142 canvas
  attributes, then updateLogicalSize:after writing 402px / 714px, with
  next-frame/transfer logs retaining 402 x 714.
• Narrowed boundary: after _ensureSize has a valid backing store and
  before/at _updateLogicalHtmlCanvasSize() logical CSS write, or a later
  overwrite of that write.

Copied collapsed-bundle style observer follow-up:

iphone_evidence_runs/20260630T024955_simulator_collapsed_bundle_canvas_style_observer_inline_layout_one_benchmark.json

• Bundle:
  diagnostic_bundles/20260629T1735_collapsed_skwasm_ready_rerun_bundle/bundle
• Result: all-benchmarks-complete
• Loaded artifacts: skwasm_heavy, main.dart.mjs, main.dart.wasm
• Profiles: one bench_card_infinite_scroll profile
• Errors: []
• Style write events: 642
• Canvas style snapshots: 32
• Key finding: the canvas itself receives inline CSS
  0.333333px / 0.333333px; the connected computed/rect value is
  0.328125 x 0.328125; the backing store later reaches 1206 x 2142, but
  the canvas CSS size is not corrected to 402 x 714.
• The repeated 402.0px / 714.0px writes in this trace appear to belong to
  outer flutter-view sizing, not the canvas element.

Analysis artifact:

diagnostic_bundles/20260630T_canvas_style_observer_trace_analysis/canvas_style_observer_analysis.md

Boundary: the diagnostic logging blocker is now narrowed to local Dart
compiler/platform-kernel/support-runtime drift and cleared for the aligned
local diagnostic control. The copied collapsed-bundle observer narrows the
remaining source question further: why does the canvas keep an early 1 / DPR
logical CSS size after the backing store is resized? This does not fix the
original Mobile Safari canvas sizing bug, and it is not default WebKit/Safari
enablement evidence.