Exposing ScrollBehaviors for app-wide settings (#76739)

packages/flutter/lib/src/cupertino/app.dart

@@ -168,6 +168,7 @@ class CupertinoApp extends StatefulWidget {
     this.shortcuts,
     this.actions,
     this.restorationScopeId,
+    this.scrollBehavior,
   }) : assert(routes != null),
        assert(navigatorObservers != null),
        assert(title != null),
@@ -207,6 +208,7 @@ class CupertinoApp extends StatefulWidget {
     this.shortcuts,
     this.actions,
     this.restorationScopeId,
+    this.scrollBehavior,
   }) : assert(title != null),
        assert(showPerformanceOverlay != null),
        assert(checkerboardRasterCacheImages != null),
@@ -393,6 +395,16 @@ class CupertinoApp extends StatefulWidget {
   /// {@macro flutter.widgets.widgetsApp.restorationScopeId}
   final String? restorationScopeId;
 
+  /// {@macro flutter.material.materialApp.scrollBehavior}
+  ///
+  /// When null, defaults to [CupertinoScrollBehavior].
+  ///
+  /// See also:
+  ///
+  ///  * [ScrollConfiguration], which controls how [Scrollable] widgets behave
+  ///    in a subtree.
+  final ScrollBehavior? scrollBehavior;
+
   @override
   _CupertinoAppState createState() => _CupertinoAppState();
 
@@ -403,7 +415,18 @@ class CupertinoApp extends StatefulWidget {
       HeroController(); // Linear tweening.
 }
 
-class _AlwaysCupertinoScrollBehavior extends ScrollBehavior {
+/// Describes how [Scrollable] widgets behave for [CupertinoApp]s.
+///
+/// {@macro flutter.widgets.scrollBehavior}
+///
+/// Setting a [CupertinoScrollBehavior] will result in descendant [Scrollable] widgets
+/// using [BouncingScrollPhysics] by default. No [GlowingOverscrollIndicator] is
+/// applied when using a [CupertinoScrollBehavior] either, regardless of platform.
+///
+/// See also:
+///
+///  * [ScrollBehavior], the default scrolling behavior extended by this class.
+class CupertinoScrollBehavior extends ScrollBehavior {
   @override
   Widget buildViewportChrome(BuildContext context, Widget child, AxisDirection axisDirection) {
     // Never build any overscroll glow indicators.
@@ -521,7 +544,7 @@ class _CupertinoAppState extends State<CupertinoApp> {
     final CupertinoThemeData effectiveThemeData = widget.theme ?? const CupertinoThemeData();
 
     return ScrollConfiguration(
-      behavior: _AlwaysCupertinoScrollBehavior(),
+      behavior: widget.scrollBehavior ?? CupertinoScrollBehavior(),
       child: CupertinoUserInterfaceLevel(
         data: CupertinoUserInterfaceLevelData.base,
         child: CupertinoTheme(

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

@@ -196,6 +196,7 @@ class MaterialApp extends StatefulWidget {
     this.shortcuts,
     this.actions,
     this.restorationScopeId,
+    this.scrollBehavior,
   }) : assert(routes != null),
        assert(navigatorObservers != null),
        assert(title != null),
@@ -242,6 +243,7 @@ class MaterialApp extends StatefulWidget {
     this.shortcuts,
     this.actions,
     this.restorationScopeId,
+    this.scrollBehavior,
   }) : assert(routeInformationParser != null),
        assert(routerDelegate != null),
        assert(title != null),
@@ -632,6 +634,23 @@ class MaterialApp extends StatefulWidget {
   /// {@macro flutter.widgets.widgetsApp.restorationScopeId}
   final String? restorationScopeId;
 
+  /// {@template flutter.material.materialApp.scrollBehavior}
+  /// The default [ScrollBehavior] for the application.
+  ///
+  /// [ScrollBehavior]s describe how [Scrollable] widgets behave. Providing
+  /// a [ScrollBehavior] can set the default [ScrollPhysics] across
+  /// an application, and manage [Scrollable] decorations like [Scrollbar]s and
+  /// [GlowingOverscrollIndicator]s.
+  /// {@endtemplate}
+  ///
+  /// When null, defaults to [MaterialScrollBehavior].
+  ///
+  /// See also:
+  ///
+  ///  * [ScrollConfiguration], which controls how [Scrollable] widgets behave
+  ///    in a subtree.
+  final ScrollBehavior? scrollBehavior;
+
   /// Turns on a [GridPaper] overlay that paints a baseline grid
   /// Material apps.
   ///
@@ -657,7 +676,18 @@ class MaterialApp extends StatefulWidget {
   }
 }
 
-class _MaterialScrollBehavior extends ScrollBehavior {
+/// Describes how [Scrollable] widgets behave for [MaterialApp]s.
+///
+/// {@macro flutter.widgets.scrollBehavior}
+///
+/// Setting a [MaterialScrollBehavior] will apply a
+/// [GlowingOverscrollIndicator] to [Scrollable] descendants when executing on
+/// [TargetPlatform.android] and [TargetPlatform.fuchsia].
+///
+/// See also:
+///
+///  * [ScrollBehavior], the default scrolling behavior extended by this class.
+class MaterialScrollBehavior extends ScrollBehavior {
   @override
   TargetPlatform getPlatform(BuildContext context) {
     return Theme.of(context).platform;
@@ -850,7 +880,7 @@ class _MaterialAppState extends State<MaterialApp> {
     }());
 
     return ScrollConfiguration(
-      behavior: _MaterialScrollBehavior(),
+      behavior: widget.scrollBehavior ?? MaterialScrollBehavior(),
       child: HeroControllerScope(
         controller: _heroController,
         child: result,

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

@@ -14,8 +14,21 @@ const Color _kDefaultGlowColor = Color(0xFFFFFFFF);
 
 /// Describes how [Scrollable] widgets should behave.
 ///
+/// {@template flutter.widgets.scrollBehavior}
 /// Used by [ScrollConfiguration] to configure the [Scrollable] widgets in a
 /// subtree.
+///
+/// This class can be extended to further customize a [ScrollBehavior] for a
+/// subtree. For example, overriding [ScrollBehavior.getScrollPhysics] sets the
+/// default [ScrollPhysics] for [Scrollable]s that inherit this [ScrollConfiguration].
+/// Overriding [ScrollBehavior.buildViewportChrome] can be used to add or change
+/// default decorations like [GlowingOverscrollIndicator]s.
+/// {@endtemplate}
+///
+/// See also:
+///
+///   * [ScrollConfiguration], the inherited widget that controls how
+///     [Scrollable] widgets behave in a subtree.
 @immutable
 class ScrollBehavior {
   /// Creates a description of how [Scrollable] widgets should behave.
@@ -33,7 +46,7 @@ class ScrollBehavior {
   /// overscrolls.
   Widget buildViewportChrome(BuildContext context, Widget child, AxisDirection axisDirection) {
     // When modifying this function, consider modifying the implementation in
-    // _MaterialScrollBehavior as well.
+    // MaterialScrollBehavior as well.
     switch (getPlatform(context)) {
       case TargetPlatform.iOS:
       case TargetPlatform.linux:

packages/flutter/test/cupertino/app_test.dart

@@ -178,6 +178,44 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('popped'), findsOneWidget);
   });
+
+  testWidgets('CupertinoApp has correct default ScrollBehavior', (WidgetTester tester) async {
+    late BuildContext capturedContext;
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Builder(
+          builder: (BuildContext context) {
+            capturedContext = context;
+            return const Placeholder();
+          },
+        ),
+      ),
+    );
+    expect(ScrollConfiguration.of(capturedContext).runtimeType, CupertinoScrollBehavior);
+  });
+
+  testWidgets('A ScrollBehavior can be set for CupertinoApp', (WidgetTester tester) async {
+    late BuildContext capturedContext;
+    await tester.pumpWidget(
+      CupertinoApp(
+        scrollBehavior: MockScrollBehavior(),
+        home: Builder(
+          builder: (BuildContext context) {
+            capturedContext = context;
+            return const Placeholder();
+          },
+        ),
+      ),
+    );
+    final ScrollBehavior scrollBehavior = ScrollConfiguration.of(capturedContext);
+    expect(scrollBehavior.runtimeType, MockScrollBehavior);
+    expect(scrollBehavior.getScrollPhysics(capturedContext).runtimeType, NeverScrollableScrollPhysics);
+  });
+}
+
+class MockScrollBehavior extends ScrollBehavior {
+  @override
+  ScrollPhysics getScrollPhysics(BuildContext context) => const NeverScrollableScrollPhysics();
 }
 
 typedef SimpleRouterDelegateBuilder = Widget Function(BuildContext, RouteInformation);

packages/flutter/test/material/app_test.dart

@@ -1026,6 +1026,44 @@ void main() {
     ));
     expect(builderChild, isNull);
   });
+
+  testWidgets('MaterialApp has correct default ScrollBehavior', (WidgetTester tester) async {
+    late BuildContext capturedContext;
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Builder(
+          builder: (BuildContext context) {
+            capturedContext = context;
+            return const Placeholder();
+          },
+        ),
+      ),
+    );
+    expect(ScrollConfiguration.of(capturedContext).runtimeType, MaterialScrollBehavior);
+  });
+
+  testWidgets('A ScrollBehavior can be set for MaterialApp', (WidgetTester tester) async {
+    late BuildContext capturedContext;
+    await tester.pumpWidget(
+      MaterialApp(
+        scrollBehavior: MockScrollBehavior(),
+        home: Builder(
+          builder: (BuildContext context) {
+            capturedContext = context;
+            return const Placeholder();
+          },
+        ),
+      ),
+    );
+    final ScrollBehavior scrollBehavior = ScrollConfiguration.of(capturedContext);
+    expect(scrollBehavior.runtimeType, MockScrollBehavior);
+    expect(scrollBehavior.getScrollPhysics(capturedContext).runtimeType, NeverScrollableScrollPhysics);
+  });
+}
+
+class MockScrollBehavior extends ScrollBehavior {
+  @override
+  ScrollPhysics getScrollPhysics(BuildContext context) => const NeverScrollableScrollPhysics();
 }
 
 class MockAccessibilityFeature implements AccessibilityFeatures {

packages/flutter/test/widgets/app_test.dart

@@ -308,6 +308,25 @@ void main() {
     ));
     expect(find.text('/'), findsOneWidget);
   });
+
+  testWidgets('WidgetsApp has correct default ScrollBehavior', (WidgetTester tester) async {
+    late BuildContext capturedContext;
+    await tester.pumpWidget(
+      WidgetsApp(
+        builder: (BuildContext context, Widget? child) {
+          capturedContext = context;
+          return const Placeholder();
+        },
+        color: const Color(0xFF123456),
+      ),
+    );
+    expect(ScrollConfiguration.of(capturedContext).runtimeType, ScrollBehavior);
+  });
+}
+
+class MockScrollBehavior extends ScrollBehavior {
+  @override
+  ScrollPhysics getScrollPhysics(BuildContext context) => const NeverScrollableScrollPhysics();
 }
 
 typedef SimpleRouterDelegateBuilder = Widget Function(BuildContext, RouteInformation);