[material_ui] Fix ExpansionTile WidgetSpan semantics assertion

[ahamSel]

On Android, embedding an ExpansionTile in a WidgetSpan can trigger a sibling-configuration conflict assertion when semantics are enabled. Its live-region Semantics uses AccessibilityFocusBlockType.blockNode but can merge into the surrounding semantics tree.

This makes the live region a semantics container, so blockNode applies to an explicit node. The Android regression test uses the simpler reproduction suggested in flutter/flutter#190639.

Fixes flutter/flutter#188298.

This ports flutter/flutter#190639 to packages/material_ui. The earlier package PR, #12383, was closed while contributions were paused. This replacement is rebased onto current main following the request to move the fix here; GitHub would not reopen the old PR after the rebase.

I used AI to help with the fix and regression test.

Pre-Review 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 page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Validation performed:

• flutter test --no-pub test/expansion_tile_test.dart: 64 passed, one pre-existing skip.
• Removing container: true makes the new regression test fail with the original !conflict assertion; restoring it makes the test pass.
• flutter analyze --no-pub lib/src/expansion_tile.dart test/expansion_tile_test.dart: no issues.
• Dart formatting and git diff --check pass.

Full repository CI is still pending. The release metadata uses material_ui's batch-release convention: a pending changelog entry with version: patch.

The repository-wide flutter_plugin_tools validate command could not complete in the local sparse checkout: its root README check requires other packages, beginning with animations, that are not checked out. This check is left to CI.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request sets container: true on the Semantics widget in ExpansionTile to ensure the live region forms the node that blockNode removes from focus traversal, resolving an Android semantics assertion when embedded in a WidgetSpan. It also adds a regression test for this scenario and a changelog entry. There are no review comments, so no feedback is provided.