Reland "fix a Scaffold.bottomSheet update bug" (#106775)

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

@@ -2124,6 +2124,17 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin, Resto
         return false;
       }
 
+      // Stop the animation and unmount the dismissed sheets from the tree immediately,
+      // otherwise may cause duplicate GlobalKey assertion if the sheet sub-tree contains
+      // GlobalKey widgets.
+      if (_dismissedBottomSheets.isNotEmpty) {
+        final List<_StandardBottomSheet> sheets = List<_StandardBottomSheet>.of(_dismissedBottomSheets, growable: false);
+        for (final _StandardBottomSheet sheet in sheets) {
+          sheet.animationController.reset();
+        }
+        assert(_dismissedBottomSheets.isEmpty);
+      }
+
       _currentBottomSheet = _buildBottomSheet<void>(
         (BuildContext context) {
           return NotificationListener<DraggableScrollableNotification>(

packages/flutter/test/material/persistent_bottom_sheet_test.dart

@@ -22,6 +22,24 @@ void main() {
     expect(dyDelta1, isNot(moreOrLessEquals(dyDelta2, epsilon: 0.1)));
   }
 
+  // Regression test for https://github.com/flutter/flutter/issues/83668
+  testWidgets('Scaffold.bottomSheet update test', (WidgetTester tester) async {
+    Widget buildFrame(Widget? bottomSheet) {
+      return MaterialApp(
+        home: Scaffold(
+          body: const Placeholder(),
+          bottomSheet: bottomSheet,
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(const Text('I love Flutter!')));
+    await tester.pumpWidget(buildFrame(null));
+
+    // The disappearing animation has not yet been completed.
+    await tester.pumpWidget(buildFrame(const Text('I love Flutter!')));
+  });
+
   testWidgets('Verify that a BottomSheet can be rebuilt with ScaffoldFeatureController.setState()', (WidgetTester tester) async {
     final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
     int buildCount = 0;