[Dismissible]Fix a state lose issue (#108971)

packages/flutter/lib/src/widgets/dismissible.dart

@@ -323,6 +323,8 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
   Size? _sizePriorToCollapse;
   bool _dismissThresholdReached = false;
 
+  final GlobalKey _contentKey = GlobalKey();
+
   @override
   bool get wantKeepAlive => (_moveController?.isAnimating ?? false) || (_resizeController?.isAnimating ?? false);
 
@@ -668,7 +670,7 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
 
     Widget content = SlideTransition(
       position: _moveAnimation,
-      child: widget.child,
+      child: KeyedSubtree(key: _contentKey, child: widget.child),
     );
 
     if (background != null) {

packages/flutter/test/material/debug_test.dart

@@ -329,6 +329,7 @@ void main() {
       '     PhysicalModel\n'
       '     AnimatedPhysicalModel\n'
       '     Material\n'
+      '     KeyedSubtree-[GlobalKey#00000]\n'
       '     FractionalTranslation\n'
       '     SlideTransition\n'
       '     Listener\n'

packages/flutter/test/widgets/dismissible_test.dart

@@ -1150,4 +1150,32 @@ void main() {
     expect(reportedDismissUpdatePreviousReached, false);
     expect(reportedDismissUpdateProgress, 0.0);
   });
+
+  testWidgets('Change direction does not lose child state', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/108961
+    Widget buildFrame(DismissDirection direction) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: Dismissible(
+          dragStartBehavior: DragStartBehavior.down,
+          direction: direction,
+          key: const Key('Dismissible'),
+          resizeDuration: null,
+          child: const SizedBox(
+            width: 100.0,
+            height: 100.0,
+            child: Text('I Love Flutter!'),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(DismissDirection.horizontal));
+    final RenderBox textRenderObjectBegin = tester.renderObject(find.text('I Love Flutter!'));
+
+    await tester.pumpWidget(buildFrame(DismissDirection.none));
+    final RenderBox textRenderObjectEnd = tester.renderObject(find.text('I Love Flutter!'));
+
+    expect(identical(textRenderObjectBegin, textRenderObjectEnd), true);
+  });
 }