[material_ui] Port PR (#187366) from flutter/flutter to material_ui

[ishivamg]

This PR ports flutter/flutter#187366 to material_ui.

When a DropdownButton menu is open (especially inside a dialog) and the device orientation changes, _DropdownButtonState.build() dismissed the route with Navigator.removeRoute() during build. That triggers setState() or markNeedsBuild() called during build.

The route is now removed synchronously unless we are in SchedulerPhase.persistentCallbacks, in which case removal is deferred to a post-frame callback.

Fixes flutter/flutter#171011
Follows flutter/flutter#188444

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 under[^1].
• 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 under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request defers the dismissal of the dropdown route during orientation changes if the scheduler is in the persistent callbacks phase, preventing Navigator mutation during the build phase. It also adds a corresponding widget test to verify this behavior and a changelog entry. There are no review comments, and I have no feedback to provide.

[navaronbracke]

LGTM with nit

[navaronbracke]

Hi @ishivamg

I recently finished up my work for splitting up a relevant test file for DropdownButton
See #12782

To avoid conflicts, I would suggest if you could create the dropdown_button_test.dart file, with just your test in?

Then I won't have a conflict on dropdown_test.dart and can also easily cherry pick your change to the dropdown_button_test.dart test file before rebasing to save myself some work :)

[ishivamg]

Hi @ishivamg

I recently finished up my work for splitting up a relevant test file for DropdownButton See #12782

To avoid conflicts, I would suggest if you could create the dropdown_button_test.dart file, with just your test in?

Then I won't have a conflict on dropdown_test.dart and can also easily cherry pick your change to the dropdown_button_test.dart test file before rebasing to save myself some work :)

Moved the regression test into dropdown_button_test.dart as suggested.

[navaronbracke]

Oh, I see see that the PR checklist is missing from this PR (it was present in the older one you ported)

I restored the checklist and checked the boxes that are definitely present. Can you cross check if you did the rest of the checklist?