Change snack bar default hitTestBehavior to deferToChild when SnackBarThemeData.insetPadding is not null (flutter#148568)

The PR changes the default value of hitTestBehavior in snack bars to `HitTestBehavior.deferToChild` when snackBarTheme.insetPadding is not null, so that widgets behind snack bars affected by the value set to insetPadding, remain interactive even while a snack bar is visible. This PR can be considered as an extension to what have been done in PR flutter#127959 which fixes the same problem but for individual snack bars with margin not being null. This PR works on the theme level.

*List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.*
flutter#148566

Author: hany-achraf

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

@@ -387,7 +387,8 @@ class SnackBar extends StatefulWidget {
 
   /// 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.
+  /// If this property is null, and [margin] is not null or [SnackBarThemeData.insetPadding] of
+  /// [ThemeData.snackBarTheme] is not null, then [HitTestBehavior.deferToChild] is used by default.
   ///
   /// Please refer to [HitTestBehavior] for a detailed explanation of every behavior.
   final HitTestBehavior? hitTestBehavior;
@@ -815,7 +816,7 @@ class _SnackBarState extends State<SnackBar> {
         key: const Key('dismissible'),
         direction: dismissDirection,
         resizeDuration: null,
-        behavior: widget.hitTestBehavior ?? (widget.margin != null ? HitTestBehavior.deferToChild : HitTestBehavior.opaque),
+        behavior: widget.hitTestBehavior ?? (widget.margin != null || snackBarTheme.insetPadding != null ? HitTestBehavior.deferToChild : HitTestBehavior.opaque),
         onDismissed: (DismissDirection direction) {
           ScaffoldMessenger.of(context).removeCurrentSnackBar(reason: SnackBarClosedReason.swipe);
         },

packages/flutter/test/material/snack_bar_test.dart

@@ -4078,6 +4078,66 @@ testWidgets('SnackBarAction backgroundColor works as a Color', (WidgetTester tes
       theme.colorScheme.inversePrimary.withOpacity(0.08),
     );
   });
+
+  testWidgets('Can interact with widgets behind SnackBar when insetPadding is set in SnackBarThemeData', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/148566.
+    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(
+      theme: ThemeData(
+        snackBarTheme: const SnackBarThemeData(
+          insetPadding: EdgeInsets.only(left: 100),
+        ),
+      ),
+      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,
+                        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);
+  });
 }
 
 /// Start test for "SnackBar dismiss test".