When maintainHintSize is false, hint is centered and aligned, it is different from the original one (#168654)

This PR changes the Hint uses topLeft alignment when maintainHintSize is
false
> code
```dart
buildTextField() {
  return const Center(
    child: TextField(
      decoration: InputDecoration(
        hintText: 'hint',
        maintainHintSize: false,
      ),
    ),
  );
}
```

Before:

<img width="416" alt="b"
src="https://github.com/user-attachments/assets/fe58c2d0-4d37-4bca-aabc-0f7d4785fb2a"
/>

After:
<img width="415" alt="a"
src="https://github.com/user-attachments/assets/096fd83a-9d8f-4cb7-be1d-c3075acbfdc0"
/>



## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

---------

Co-authored-by: 郑泽钦 <zhengzeqin@addcn.com>

packages/flutter/lib/src/material/input_decorator.dart

@@ -2267,6 +2267,10 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
     return FadeTransition(opacity: _curvedAnimation!, child: child);
   }
 
+  static Widget _topStartLayout(Widget? currentChild, List<Widget> previousChildren) {
+    return Stack(children: <Widget>[...previousChildren, if (currentChild != null) currentChild]);
+  }
+
   @override
   Widget build(BuildContext context) {
     final ThemeData themeData = Theme.of(context);
@@ -2308,6 +2312,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
               : AnimatedSwitcher(
                 duration: decoration.hintFadeDuration ?? _kHintFadeTransitionDuration,
                 transitionBuilder: _buildTransition,
+                layoutBuilder: _topStartLayout,
                 child: showHint ? hintWidget : const SizedBox.shrink(),
               );
     }

packages/flutter/test/material/input_decorator_test.dart

@@ -5271,6 +5271,45 @@ void main() {
       // The hintText replaced with SizeBox.
       expect(find.text(hintText), findsNothing);
     });
+
+    testWidgets('Hint does not change position when maintainHintSize is false - LTR', (
+      WidgetTester tester,
+    ) async {
+      await tester.pumpWidget(
+        buildInputDecorator(isEmpty: true, decoration: const InputDecoration(hintText: hintText)),
+      );
+      final double expectedHintLeft = getHintRect(tester).left;
+
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          decoration: const InputDecoration(hintText: hintText, maintainHintSize: false),
+        ),
+      );
+      expect(getHintRect(tester).left, expectedHintLeft);
+    });
+
+    testWidgets('Hint does not change position when maintainHintSize is false - RTL', (
+      WidgetTester tester,
+    ) async {
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          decoration: const InputDecoration(hintText: hintText),
+          textDirection: TextDirection.rtl,
+        ),
+      );
+      final double expectedHintRight = getHintRect(tester).right;
+
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          decoration: const InputDecoration(hintText: hintText, maintainHintSize: false),
+          textDirection: TextDirection.rtl,
+        ),
+      );
+      expect(getHintRect(tester).right, expectedHintRight);
+    });
   });
 
   group('Material3 - InputDecoration helper/counter/error', () {