Tap on button behind snack bar defined by margin (#127959)

If the margin is used, set the `HitTestBehavior` to `deferToChild`. 

*List which issues are fixed by this PR. You must list at least one issue.*
#78537 
#114810 

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

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

@@ -283,6 +283,7 @@ class SnackBar extends StatefulWidget {
     this.padding,
     this.width,
     this.shape,
+    this.hitTestBehavior,
     this.behavior,
     this.action,
     this.actionOverflowThreshold,
@@ -331,6 +332,8 @@ class SnackBar extends StatefulWidget {
   /// If this property is null, then [SnackBarThemeData.insetPadding] of
   /// [ThemeData.snackBarTheme] is used. If that is also null, then the default is
   /// `EdgeInsets.fromLTRB(15.0, 5.0, 15.0, 10.0)`.
+  ///
+  /// If this property is not null and [hitTestBehavior] is null, then [hitTestBehavior] default is [HitTestBehavior.deferToChild].
   final EdgeInsetsGeometry? margin;
 
   /// The amount of padding to apply to the snack bar's content and optional
@@ -384,6 +387,13 @@ class SnackBar extends StatefulWidget {
   /// circular corner radius of 4.0.
   final ShapeBorder? shape;
 
+  /// Defines how the snack bar area, including margin, will behave during hit testing.
+  ///
+  /// If this property is null and [margin] is not null, then [HitTestBehavior.deferToChild] is used by default.
+  ///
+  /// Please refer to [HitTestBehavior] for a detailed explanation of every behavior.
+  final HitTestBehavior? hitTestBehavior;
+
   /// This defines the behavior and location of the snack bar.
   ///
   /// Defines where a [SnackBar] should appear within a [Scaffold] and how its
@@ -489,6 +499,7 @@ class SnackBar extends StatefulWidget {
       padding: padding,
       width: width,
       shape: shape,
+      hitTestBehavior: hitTestBehavior,
       behavior: behavior,
       action: action,
       actionOverflowThreshold: actionOverflowThreshold,
@@ -776,6 +787,7 @@ class _SnackBarState extends State<SnackBar> {
         key: const Key('dismissible'),
         direction: widget.dismissDirection,
         resizeDuration: null,
+        behavior: widget.hitTestBehavior ?? (widget.margin != null ? HitTestBehavior.deferToChild : HitTestBehavior.opaque),
         onDismissed: (DismissDirection direction) {
           ScaffoldMessenger.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.swipe);
         },

packages/flutter/test/material/snack_bar_test.dart

@@ -7,6 +7,7 @@
 @Tags(<String>['reduced-test-set'])
 library;
 
+import 'dart:async';
 import 'dart:ui';
 
 import 'package:flutter/material.dart';
@@ -2915,6 +2916,175 @@ testWidgets('SnackBarAction backgroundColor works as a Color', (WidgetTester tes
 
     expect(material.clipBehavior, Clip.antiAlias);
   });
+
+ testWidgets('Tap on button behind snack bar defined by width', (WidgetTester tester) async {
+    tester.view.physicalSize = const Size.square(200);
+    tester.view.devicePixelRatio = 1;
+    addTearDown(tester.view.resetPhysicalSize);
+    addTearDown(tester.view.resetDevicePixelRatio);
+
+    const String buttonText = 'Show snackbar';
+    const String snackbarContent = 'Snackbar';
+    const String buttonText2 = 'Try press me';
+
+    final Completer<void> completer = Completer<void>();
+
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return Column(
+              mainAxisAlignment: MainAxisAlignment.spaceBetween,
+              crossAxisAlignment: CrossAxisAlignment.stretch,
+              children: <Widget>[
+                ElevatedButton(
+                  onPressed: () {
+                    ScaffoldMessenger.of(context).showSnackBar(
+                      const SnackBar(
+                        behavior: SnackBarBehavior.floating,
+                        width: 100,
+                        content: Text(snackbarContent),
+                      ),
+                    );
+                  },
+                  child: const Text(buttonText),
+                ),
+                ElevatedButton(
+                  onPressed: () {
+                    completer.complete();
+                  },
+                  child: const Text(buttonText2),
+                ),
+              ],
+            );
+          },
+        ),
+      ),
+    ));
+
+    await tester.tap(find.text(buttonText));
+    await tester.pumpAndSettle();
+
+    expect(find.text(snackbarContent), findsOneWidget);
+    await tester.tapAt(tester.getTopLeft(find.text(buttonText2)));
+    expect(find.text(snackbarContent), findsOneWidget);
+
+    expect(completer.isCompleted, true);
+  });
+
+
+  testWidgets('Tap on button behind snack bar defined by margin', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/78537.
+    tester.view.physicalSize = const Size.square(200);
+    tester.view.devicePixelRatio = 1;
+    addTearDown(tester.view.resetPhysicalSize);
+    addTearDown(tester.view.resetDevicePixelRatio);
+
+    const String buttonText = 'Show snackbar';
+    const String snackbarContent = 'Snackbar';
+    const String buttonText2 = 'Try press me';
+
+    final Completer<void> completer = Completer<void>();
+
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return Column(
+              mainAxisAlignment: MainAxisAlignment.spaceBetween,
+              crossAxisAlignment: CrossAxisAlignment.stretch,
+              children: <Widget>[
+                ElevatedButton(
+                  onPressed: () {
+                    ScaffoldMessenger.of(context).showSnackBar(
+                      const SnackBar(
+                        behavior: SnackBarBehavior.floating,
+                        margin: EdgeInsets.only(left: 100),
+                        content: Text(snackbarContent),
+                      ),
+                    );
+                  },
+                  child: const Text(buttonText),
+                ),
+                ElevatedButton(
+                  onPressed: () {
+                    completer.complete();
+                  },
+                  child: const Text(buttonText2),
+                ),
+              ],
+            );
+          },
+        ),
+      ),
+    ));
+
+    await tester.tap(find.text(buttonText));
+    await tester.pumpAndSettle();
+
+    expect(find.text(snackbarContent), findsOneWidget);
+    await tester.tapAt(tester.getTopLeft(find.text(buttonText2)));
+    expect(find.text(snackbarContent), findsOneWidget);
+
+    expect(completer.isCompleted, true);
+  });
+
+  testWidgets("Can't tap on button behind snack bar defined by margin and HitTestBehavior.opaque", (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/78537.
+    tester.view.physicalSize = const Size.square(200);
+    tester.view.devicePixelRatio = 1;
+    addTearDown(tester.view.resetPhysicalSize);
+    addTearDown(tester.view.resetDevicePixelRatio);
+
+    const String buttonText = 'Show snackbar';
+    const String snackbarContent = 'Snackbar';
+    const String buttonText2 = 'Try press me';
+
+    final Completer<void> completer = Completer<void>();
+
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Builder(
+          builder: (BuildContext context) {
+            return Column(
+              mainAxisAlignment: MainAxisAlignment.spaceBetween,
+              crossAxisAlignment: CrossAxisAlignment.stretch,
+              children: <Widget>[
+                ElevatedButton(
+                  onPressed: () {
+                    ScaffoldMessenger.of(context).showSnackBar(
+                      const SnackBar(
+                        hitTestBehavior: HitTestBehavior.opaque,
+                        behavior: SnackBarBehavior.floating,
+                        margin: EdgeInsets.only(left: 100),
+                        content: Text(snackbarContent),
+                      ),
+                    );
+                  },
+                  child: const Text(buttonText),
+                ),
+                ElevatedButton(
+                  onPressed: () {
+                    completer.complete();
+                  },
+                  child: const Text(buttonText2),
+                ),
+              ],
+            );
+          },
+        ),
+      ),
+    ));
+
+    await tester.tap(find.text(buttonText));
+    await tester.pumpAndSettle();
+
+    expect(find.text(snackbarContent), findsOneWidget);
+    await tester.tapAt(tester.getTopLeft(find.text(buttonText2)));
+    expect(find.text(snackbarContent), findsOneWidget);
+
+    expect(completer.isCompleted, false);
+  });
 }
 
 /// Start test for "SnackBar dismiss test".