ScaffoldMessenger only shows to root nested Scaffold (#74093)

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

@@ -68,6 +68,12 @@ enum _ScaffoldSlot {
 /// [BuildContext] via [ScaffoldMessenger.of] and use the
 /// [ScaffoldMessengerState.showSnackBar] function.
 ///
+/// When the [ScaffoldMessenger] has nested [Scaffold] descendants, the
+/// ScaffoldMessenger will only present a [SnackBar] to the root Scaffold of
+/// the subtree of Scaffolds. In order to show SnackBars in the inner, nested
+/// Scaffolds, set a new scope for your SnackBars by instantiating a new
+/// ScaffoldMessenger in between the levels of nesting.
+///
 /// {@tool dartpad --template=stateless_widget_scaffold_center}
 ///
 /// Here is an example of showing a [SnackBar] when the user presses a button.
@@ -377,10 +383,18 @@ class ScaffoldMessengerState extends State<ScaffoldMessenger> with TickerProvide
 
   void _updateScaffolds() {
     for (final ScaffoldState scaffold in _scaffolds) {
-      scaffold._updateSnackBar();
+      if (_isRoot(scaffold))
+        scaffold._updateSnackBar();
     }
   }
 
+  // Nested Scaffolds are handled by the ScaffoldMessenger by only presenting a
+  // SnackBar in the root Scaffold of the nested set.
+  bool _isRoot(ScaffoldState scaffold) {
+    final ScaffoldState? parent = scaffold.context.findAncestorStateOfType<ScaffoldState>();
+    return parent == null || !_scaffolds.contains(parent);
+  }
+
   /// Removes the current [SnackBar] (if any) immediately from registered
   /// [Scaffold]s.
   ///

packages/flutter/test/material/snack_bar_test.dart

@@ -2291,4 +2291,36 @@ void main() {
 
     await expectLater(find.byType(MaterialApp), matchesGoldenFile('snack_bar.goldenTest.workWithBottomSheet.png'));
   });
+
+  testWidgets('ScaffoldMessenger presents SnackBars to only the root Scaffold when Scaffolds are nested.', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: const Scaffold(),
+        floatingActionButton: FloatingActionButton(onPressed: () {}),
+      ),
+    ));
+
+    final ScaffoldMessengerState scaffoldMessengerState = tester.state<ScaffoldMessengerState>(
+      find.byType(ScaffoldMessenger),
+    );
+    scaffoldMessengerState.showSnackBar(SnackBar(
+      content: const Text('ScaffoldMessenger'),
+      duration: const Duration(seconds: 2),
+      action: SnackBarAction(label: 'ACTION', onPressed: () {}),
+      behavior: SnackBarBehavior.floating,
+    ));
+    await tester.pumpAndSettle();
+
+    expect(find.byType(SnackBar), findsOneWidget);
+    // The FloatingActionButton helps us identify which Scaffold has the
+    // SnackBar here. Since the outer Scaffold contains a FAB, the SnackBar
+    // should be above it. If the inner Scaffold had the SnackBar, it would be
+    // overlapping the FAB.
+    await expectLater(
+      find.byType(MaterialApp),
+      matchesGoldenFile('snack_bar.scaffold.nested.png'),
+    );
+    final Offset snackBarTopRight = tester.getTopRight(find.byType(SnackBar));
+    expect(snackBarTopRight.dy, 465.0);
+  });
 }