[web] switch from .didGain/LoseAccessibilityFocus to .focus (#53360)

This is a repeat of https://github.com/flutter/engine/pull/53134, which was merged prematurely.

> [!WARNING]  
> Only land this after:
> * https://github.com/flutter/flutter/pull/149840 lands in the framework.
> * You have personally manually tested the change together with the latest framework on all browsers.

## Original PR description

Stop using `SemanticsAction.didGain/LoseAccessibilityFocus` on the web, start using `SemanticsAction.focus`. This is because on the web, a11y focus is not observable, only input focus is. Sending `SemanticsAction.focus` will guarantee that the framework move focus to the respective widget. There currently is no "unfocus" signal, because it seems to be already covered: either another widget gains focus, or an HTML DOM element outside the Flutter view does, both of which have their respective signals already.

More details in the discussion in the issue https://github.com/flutter/flutter/issues/83809.

Fixes https://github.com/flutter/flutter/issues/83809
Fixes https://github.com/flutter/flutter/issues/148285
Fixes https://github.com/flutter/flutter/issues/143337
9 files changed
tree: 7d3be1116571d5f2ec64564fc607d55e0066e471
  1. .github/
  2. assets/
  3. benchmarking/
  4. bin/
  5. build/
  6. build_overrides/
  7. ci/
  8. common/
  9. display_list/
  10. docs/
  11. examples/
  12. flow/
  13. flutter_frontend_server/
  14. flutter_vma/
  15. fml/
  16. impeller/
  17. lib/
  18. runtime/
  19. shell/
  20. skia/
  21. sky/
  22. testing/
  23. third_party/
  24. tools/
  25. vulkan/
  26. wasm/
  27. web_sdk/
  28. .ci.yaml
  29. .clang-format
  30. .clang-tidy
  31. .clangd
  32. .engine-release.version
  33. .gitattributes
  34. .gitignore
  35. .pylintrc
  36. .style.yapf
  37. analysis_options.yaml
  38. AUTHORS
  39. BUILD.gn
  40. CODEOWNERS
  41. CONTRIBUTING.md
  42. DEPS
  43. Doxyfile
  44. LICENSE
  45. README.md
README.md

Flutter Engine

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