Preserve SelectableRegion scope across context menu rebuilds

[mvincentong]

SelectableRegion conditionally wraps its selection subtree in PlatformSelectableRegionContextMenu on desktop web. When BrowserContextMenu.enabled changes, that wrapper can be removed while the existing root SelectionContainer is still registered, causing a replacement container to register before the old one unregisters.

This keeps the selection status scope keyed from SelectableRegionState, so the selection subtree is reparented across the optional wrapper change instead of recreated. The browser regression covers enable/disable transitions with the widgets-layer test harness and current selection controls.

Fixes#186459.

Tests:

• ../../bin/flutter test --no-pub --platform chrome test/widgets/selectable_region_context_menu_test.dart
• ./bin/flutter analyze --no-pub packages/flutter/lib/src/widgets/selectable_region.dart packages/flutter/test/widgets/selectable_region_context_menu_test.dart
• ./bin/dart format --output=none --set-exit-if-changed packages/flutter/test/widgets/selectable_region_context_menu_test.dart
• git diff --check

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request introduces a GlobalKey to SelectableRegionState to maintain the SelectableRegionSelectionStatusScope subtree across rebuilds. It also adds a widget test to ensure SelectableRegion can rebuild on the web after disabling the browser context menu. Feedback indicates that the new private GlobalKey should include public-quality documentation and a debugLabel to align with the Flutter style guide.

[Renzo-Olivares]

nit: I'd add a pump between each of these, and end with a pumpAndSettle.

[Renzo-Olivares]

Thanks for addressing this, I think it would be easier to read if this just calls BrowserContextMenu.disable/enable() directly with a pump in between rather than using this helper method.

[Renzo-Olivares]

Left a few small comments.

[Renzo-Olivares]

Hi @mvincentong, friendly ping is this something you still want to work on?

[mvincentong]

Yes—thanks for the ping. I pushed 96b9f77: the regression now uses TestWidgetsApp and testTextSelectionHandleControls, pumps each toggle with a final pumpAndSettle, and links #186459.

[Renzo-Olivares]

LGTM modulo my one comment. This will need a secondary review before landing.

[Renzo-Olivares]

Thanks for addressing this, I think it would be easier to read if this just calls BrowserContextMenu.disable/enable() directly with a pump in between rather than using this helper method.

[Piinks]

I think this will also need a rebase to resolve the ci failure.