[material_ui] Hit test the floating label of InputDecorator at its painted position - #13326

When the label of an InputDecorator floats, _RenderDecoration.paint draws the label with _labelTransform (a translation and a scale of 0.75). applyPaintTransform reports the same transform. But hitTestChildren hit tests the label with only its layout offset. A comment there says "The label must be handled specially since we've transformed it", but the code does not do this. Thus the label is painted in one location and hit tested in a different location:

• A tap on the visible floating label does not hit the label.
• A tap below the visible label, inside its unscaled layout box, hits the label.

This is a problem when the label contains an interactive widget, for example a PopupMenuButton, as in the issue. A maintainer said in the issue: "we shouldn't have weird issues with the hit testing here. Flutter should be able to do this" (comment).

This change hit tests the label with the transform from applyPaintTransform when _labelTransform is set. The hit test order of the children does not change. Other children keep the offset-based hit test. There is no API change.

The code in packages/flutter/lib/src/material of flutter/flutter is frozen (see flutter/flutter#188444), so this change is only for material_ui.

Fixes flutter/flutter#145324

Test plan

• Added Floating label hit test matches its painted position to test/input_decorator_test.dart, for TextDirection.ltr and TextDirection.rtl. The test taps the painted label two times (2 taps expected), then taps 10 px below the painted label, inside its layout box (no new tap expected).
  • Without the fix: both cases fail with Expected: <2> Actual: <0>.
  • With the fix: both cases pass.
• flutter test test/input_decorator_test.dart (+555, all passed)
• flutter test test/text_field_test.dart test/text_form_field_test.dart test/dropdown_menu_test.dart (all passed)
• flutter analyze lib/src/input_decorator.dart test/input_decorator_test.dart (no issues)
• dart format --output=none --set-exit-if-changed lib/src/input_decorator.dart test/input_decorator_test.dart (0 changed)

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.

The change uses the repository's pending changelog workflow (packages/material_ui/pending_changelogs/change_2026_10_20_floating_label_hit_test.yaml, version: patch). No public API documentation change is necessary.

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