Update Scrollbar behavior for mobile devices (#72531)

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

@@ -18,18 +18,18 @@ const Radius _kScrollbarRadius = Radius.circular(8.0);
 const Duration _kScrollbarFadeDuration = Duration(milliseconds: 300);
 const Duration _kScrollbarTimeToFade = Duration(milliseconds: 600);
 
-/// A material design scrollbar.
+/// A Material Design scrollbar.
 ///
-/// To add a scrollbar thumb to a [ScrollView], simply wrap the scroll view
+/// To add a scrollbar to a [ScrollView], wrap the scroll view
 /// widget in a [Scrollbar] widget.
 ///
 /// {@macro flutter.widgets.Scrollbar}
 ///
-/// The color of the Scrollbar will change when dragged, as well as when
-/// hovered over. A scrollbar track can also been drawn when triggered by a
-/// hover event, which is controlled by [showTrackOnHover]. The thickness of the
-/// track and scrollbar thumb will become larger when hovering, unless
-/// overridden by [hoverThickness].
+/// The color of the Scrollbar will change when dragged. A hover animation is
+/// also triggered when used on web and desktop platforms. A scrollbar track
+/// can also been drawn when triggered by a hover event, which is controlled by
+/// [showTrackOnHover]. The thickness of the track and scrollbar thumb will
+/// become larger when hovering, unless overridden by [hoverThickness].
 ///
 // TODO(Piinks): Add code sample
 ///
@@ -50,8 +50,11 @@ class Scrollbar extends RawScrollbar {
   /// If the [controller] is null, the default behavior is to
   /// enable scrollbar dragging using the [PrimaryScrollController].
   ///
-  /// When null, [thickness] and [radius] defaults will result in a rounded
-  /// rectangular thumb that is 8.0 dp wide with a radius of 8.0 pixels.
+  /// When null, [thickness] defaults to 8.0 pixels on desktop and web, and 4.0
+  /// pixels when on mobile platforms. A null [radius] will result in a default
+  /// of an 8.0 pixel circular radius about the corners of the scrollbar thumb,
+  /// except for when executing on [TargetPlatform.android], which will render the
+  /// thumb without a radius.
   const Scrollbar({
     Key? key,
     required Widget child,
@@ -66,7 +69,7 @@ class Scrollbar extends RawScrollbar {
          child: child,
          controller: controller,
          isAlwaysShown: isAlwaysShown,
-         thickness: thickness ?? _kScrollbarThickness,
+         thickness: thickness,
          radius: radius,
          fadeDuration: _kScrollbarFadeDuration,
          timeToFade: _kScrollbarTimeToFade,
@@ -93,6 +96,11 @@ class _ScrollbarState extends RawScrollbarState<Scrollbar> {
   bool _dragIsActive = false;
   bool _hoverIsActive = false;
   late ColorScheme _colorScheme;
+  // On Android, scrollbars should match native appearance.
+  late bool _useAndroidScrollbar;
+  // Hover events should be ignored on mobile, the exit event cannot be
+  // triggered, but the enter event can on tap.
+  late bool _isMobile;
 
   Set<MaterialState> get _states => <MaterialState>{
     if (_dragIsActive) MaterialState.dragged,
@@ -165,7 +173,8 @@ class _ScrollbarState extends RawScrollbarState<Scrollbar> {
     return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
       if (states.contains(MaterialState.hovered) && widget.showTrackOnHover)
         return widget.hoverThickness ?? _kScrollbarThicknessWithTrack;
-      return widget.thickness ?? _kScrollbarThickness;
+      // The default scrollbar thickness is smaller on mobile.
+      return widget.thickness ?? (_kScrollbarThickness / (_isMobile ? 2 : 1));
     });
   }
 
@@ -181,6 +190,29 @@ class _ScrollbarState extends RawScrollbarState<Scrollbar> {
     });
   }
 
+  @override
+  void didChangeDependencies() {
+    final ThemeData theme = Theme.of(context);
+    switch (theme.platform) {
+      case TargetPlatform.android:
+        _useAndroidScrollbar = true;
+        _isMobile = true;
+        break;
+      case TargetPlatform.iOS:
+        _useAndroidScrollbar = false;
+        _isMobile = true;
+        break;
+      case TargetPlatform.linux:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.macOS:
+      case TargetPlatform.windows:
+        _useAndroidScrollbar = false;
+        _isMobile = false;
+        break;
+    }
+    super.didChangeDependencies();
+  }
+
   @override
   void updateScrollbarPainter() {
     _colorScheme = Theme.of(context).colorScheme;
@@ -190,8 +222,8 @@ class _ScrollbarState extends RawScrollbarState<Scrollbar> {
       ..trackBorderColor = _trackBorderColor.resolve(_states)
       ..textDirection = Directionality.of(context)
       ..thickness = _thickness.resolve(_states)
-      ..radius = widget.radius ?? _kScrollbarRadius
-      ..crossAxisMargin = _kScrollbarMargin
+      ..radius = widget.radius ?? (_useAndroidScrollbar ? null : _kScrollbarRadius)
+      ..crossAxisMargin = (_useAndroidScrollbar ? 0.0 : _kScrollbarMargin)
       ..minLength = _kScrollbarMinLength
       ..padding = MediaQuery.of(context).padding;
   }
@@ -210,6 +242,8 @@ class _ScrollbarState extends RawScrollbarState<Scrollbar> {
 
   @override
   void handleHover(PointerHoverEvent event) {
+    // Hover events should not be triggered on mobile.
+    assert(!_isMobile);
     super.handleHover(event);
     // Check if the position of the pointer falls over the painted scrollbar
     if (isPointerOverScrollbar(event.position)) {
@@ -225,6 +259,8 @@ class _ScrollbarState extends RawScrollbarState<Scrollbar> {
 
   @override
   void handleHoverExit(PointerExitEvent event) {
+    // Hover events should not be triggered on mobile.
+    assert(!_isMobile);
     super.handleHoverExit(event);
     setState(() { _hoverIsActive = false; });
     _hoverAnimationController.reverse();

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

@@ -781,6 +781,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   late Animation<double> _fadeoutOpacityAnimation;
   final GlobalKey  _scrollbarPainterKey = GlobalKey();
   bool _hoverIsActive = false;
+  late bool _isMobile;
 
 
   /// Used to paint the scrollbar.
@@ -811,6 +812,18 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.android:
+      case TargetPlatform.iOS:
+        _isMobile = true;
+        break;
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.macOS:
+      case TargetPlatform.windows:
+        _isMobile = false;
+        break;
+    }
     _maybeTriggerScrollbar();
   }
 
@@ -1145,20 +1158,28 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @override
   Widget build(BuildContext context) {
     updateScrollbarPainter();
+
+    Widget child = CustomPaint(
+      key: _scrollbarPainterKey,
+      foregroundPainter: scrollbarPainter,
+      child: RepaintBoundary(child: widget.child),
+    );
+
+    if (!_isMobile) {
+      // Hover events not supported on mobile.
+      child = MouseRegion(
+        onExit: handleHoverExit,
+        onHover: handleHover,
+        child: child
+      );
+    }
+
     return NotificationListener<ScrollNotification>(
       onNotification: _handleScrollNotification,
       child: RepaintBoundary(
         child: RawGestureDetector(
           gestures: _gestures,
-          child: MouseRegion(
-            onExit: handleHoverExit,
-            onHover: handleHover,
-            child: CustomPaint(
-              key: _scrollbarPainterKey,
-              foregroundPainter: scrollbarPainter,
-              child: RepaintBoundary(child: widget.child),
-            ),
-          ),
+          child: child,
         ),
       ),
     );

packages/flutter/test/material/scrollbar_paint_test.dart

@@ -30,7 +30,24 @@ void main() {
     ));
     expect(find.byType(Scrollbar), isNot(paints..rect()));
     await tester.fling(find.byType(SingleChildScrollView), const Offset(0.0, -10.0), 10.0);
-    expect(find.byType(Scrollbar), paints..rect(rect: const Rect.fromLTRB(800.0 - 12.0, 0.0, 800.0, 600.0)));
+    expect(
+      find.byType(Scrollbar),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 1.5, 800.0, 91.5),
+          color: const Color(0x1a000000),
+        ),
+    );
   });
 
   testWidgets('Viewport basic test (RTL)', (WidgetTester tester) async {
@@ -40,7 +57,24 @@ void main() {
     ));
     expect(find.byType(Scrollbar), isNot(paints..rect()));
     await tester.fling(find.byType(SingleChildScrollView), const Offset(0.0, -10.0), 10.0);
-    expect(find.byType(Scrollbar), paints..rect(rect: const Rect.fromLTRB(0.0, 0.0, 12.0, 600.0)));
+    expect(
+      find.byType(Scrollbar),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(0.0, 0.0, 4.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(0.0, 0.0),
+          p2: const Offset(0.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(0.0, 1.5, 4.0, 91.5),
+          color: const Color(0x1a000000),
+        ),
+    );
   });
 
   testWidgets('works with MaterialApp and Scaffold', (WidgetTester tester) async {
@@ -67,15 +101,24 @@ void main() {
     await tester.pump();
     await tester.pump(const Duration(milliseconds: 500));
 
-    expect(find.byType(Scrollbar), paints..rect(
-      rect: const Rect.fromLTWH(
-        800.0 - 12, // screen width - default thickness and margin
-        0,         // the paint area starts from the bottom of the app bar
-        12,         // thickness
-        // 56 being the height of the app bar
-        600.0 - 56 - 34 - 20,
-      ),
-    ));
+    expect(
+      find.byType(Scrollbar),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 490.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 490.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTWH(796.0, 0.0, 4.0, (600.0 - 56 - 34 - 20) / 4000 * (600 - 56 - 34 - 20)),
+          color: const Color(0x1a000000),
+        ),
+    );
   });
 
   testWidgets("should not paint when there isn't enough space", (WidgetTester tester) async {

packages/flutter/test/material/scrollbar_test.dart

@@ -502,12 +502,27 @@ void main() {
     await tester.pump();
 
     // Long press on the scrollbar thumb and expect it to grow
-    expect(find.byType(Scrollbar), paints..rrect(
-      rrect: RRect.fromRectAndRadius(const Rect.fromLTWH(778, 0, 20, 300), const Radius.circular(8)),
-    ));
+    expect(
+      find.byType(Scrollbar),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(780.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(780.0, 0.0),
+          p2: const Offset(780.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(780.0, 0.0, 800.0, 300.0),
+          color: const Color(0x1a000000),
+        ),
+    );
     await tester.pumpWidget(viewWithScroll(radius: const Radius.circular(10)));
     expect(find.byType(Scrollbar), paints..rrect(
-      rrect: RRect.fromRectAndRadius(const Rect.fromLTWH(778, 0, 20, 300), const Radius.circular(10)),
+      rrect: RRect.fromRectAndRadius(const Rect.fromLTRB(780, 0.0, 800.0, 300.0), const Radius.circular(10)),
     ));
 
     await tester.pumpAndSettle();
@@ -536,9 +551,21 @@ void main() {
     expect(scrollController.offset, 0.0);
     expect(
       find.byType(Scrollbar),
-      paints..rrect(
-        rrect: RRect.fromLTRBR(790.0, 0.0, 798.0, 360.0, const Radius.circular(8.0)),
-      )
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 360.0),
+          color: const Color(0x1a000000),
+        ),
     );
 
     // Tap on the track area below the thumb.
@@ -548,12 +575,21 @@ void main() {
     expect(scrollController.offset, 400.0);
     expect(
       find.byType(Scrollbar),
-      paints..rrect(
-        rrect: RRect.fromRectAndRadius(
-          const Rect.fromLTRB(790.0, 240.0, 798.0, 600.0),
-          const Radius.circular(8.0),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 240.0, 800.0, 600.0),
+          color: const Color(0x1a000000),
         ),
-      )
     );
 
     // Tap on the track area above the thumb.
@@ -563,9 +599,21 @@ void main() {
     expect(scrollController.offset, 0.0);
     expect(
       find.byType(Scrollbar),
-      paints..rrect(
-        rrect: RRect.fromLTRBR(790.0, 0.0, 798.0, 360.0, const Radius.circular(8.0)),
-      )
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 360.0),
+          color: const Color(0x1a000000),
+        ),
     );
   });
 
@@ -586,13 +634,21 @@ void main() {
     await tester.pump(const Duration(milliseconds: 500));
     expect(
       find.byType(Scrollbar),
-      paints..rrect(
-        rrect: RRect.fromRectAndRadius(
-          const Rect.fromLTRB(790.0, 3.0, 798.0, 93.0),
-          const Radius.circular(8.0),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 3.0, 800.0, 93.0),
+          color: const Color(0x1a000000),
         ),
-        color: const Color(0x1a000000),
-      ),
     );
 
     await tester.pump(const Duration(seconds: 3));
@@ -600,13 +656,21 @@ void main() {
     // Still there.
     expect(
       find.byType(Scrollbar),
-      paints..rrect(
-        rrect: RRect.fromRectAndRadius(
-          const Rect.fromLTRB(790.0, 3.0, 798.0, 93.0),
-          const Radius.circular(8.0),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 3.0, 800.0, 93.0),
+          color: const Color(0x1a000000),
         ),
-        color: const Color(0x1a000000),
-      ),
     );
 
     await gesture.up();
@@ -616,13 +680,21 @@ void main() {
     // Opacity going down now.
     expect(
       find.byType(Scrollbar),
-      paints..rrect(
-        rrect: RRect.fromRectAndRadius(
-          const Rect.fromLTRB(790.0, 3.0, 798.0, 93.0),
-          const Radius.circular(8.0),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 3.0, 800.0, 93.0),
+          color: const Color(0x14000000),
         ),
-        color: const Color(0x14000000),
-      ),
     );
   });
 
@@ -646,13 +718,21 @@ void main() {
     expect(scrollController.offset, 0.0);
     expect(
       find.byType(Scrollbar),
-      paints..rrect(
-        rrect: RRect.fromRectAndRadius(
-          const Rect.fromLTRB(790.0, 0.0, 798.0, 90.0),
-          const Radius.circular(8.0),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 90.0),
+          color: const Color(0x1a000000),
         ),
-        color: const Color(0x1a000000),
-      ),
     );
 
     // Drag the thumb down to scroll down.
@@ -662,14 +742,22 @@ void main() {
 
     expect(
       find.byType(Scrollbar),
-      paints..rrect(
-        rrect: RRect.fromRectAndRadius(
-          const Rect.fromLTRB(790.0, 0.0, 798.0, 90.0),
-          const Radius.circular(8.0),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 90.0),
+          // Drag color
+          color: const Color(0x99000000),
         ),
-        // Drag color
-        color: const Color(0x99000000),
-      ),
     );
 
     await dragScrollbarGesture.moveBy(const Offset(0.0, scrollAmount));
@@ -682,13 +770,21 @@ void main() {
     expect(scrollController.offset, greaterThan(scrollAmount * 2));
     expect(
       find.byType(Scrollbar),
-      paints..rrect(
-        rrect: RRect.fromRectAndRadius(
-          const Rect.fromLTRB(790.0, 10.0, 798.0, 100.0),
-          const Radius.circular(8.0),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 0.0, 800.0, 600.0),
+          color: const Color(0x00000000),
+        )
+        ..line(
+          p1: const Offset(796.0, 0.0),
+          p2: const Offset(796.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x00000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(796.0, 10.0, 800.0, 100.0),
+          color: const Color(0x1a000000),
         ),
-        color: const Color(0x1a000000),
-      ),
     );
   });
 
@@ -737,7 +833,63 @@ void main() {
         color: const Color(0x80000000),
       ),
     );
-  });
+  },
+    variant: const TargetPlatformVariant(<TargetPlatform>{
+      TargetPlatform.linux,
+      TargetPlatform.macOS,
+      TargetPlatform.windows,
+      TargetPlatform.fuchsia,
+    }),
+  );
+
+  testWidgets('Hover animation is not triggered on mobile', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      MaterialApp(
+        home: PrimaryScrollController(
+          controller: scrollController,
+          child: Scrollbar(
+            isAlwaysShown: true,
+            showTrackOnHover: true,
+            controller: scrollController,
+            child: const SingleChildScrollView(
+              child: SizedBox(width: 4000.0, height: 4000.0)
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(
+      find.byType(Scrollbar),
+      paints..rrect(
+        rrect: RRect.fromRectAndRadius(
+          const Rect.fromLTRB(794.0, 0.0, 798.0, 90.0),
+          const Radius.circular(8.0),
+        ),
+        color: const Color(0x1a000000),
+      ),
+    );
+    await tester.tapAt(const Offset(794.0, 5.0));
+    await tester.pumpAndSettle();
+
+    // Tapping on mobile triggers a hover enter event. In this case, the
+    // Scrollbar should be unchanged since it ignores hover events on mobile.
+    expect(
+      find.byType(Scrollbar),
+      paints..rrect(
+        rrect: RRect.fromRectAndRadius(
+          const Rect.fromLTRB(794.0, 0.0, 798.0, 90.0),
+          const Radius.circular(8.0),
+        ),
+        color: const Color(0x1a000000),
+      ),
+    );
+  },
+    variant: const TargetPlatformVariant(<TargetPlatform>{
+      TargetPlatform.iOS,
+    }),
+  );
 
   testWidgets('Scrollbar showTrackOnHover', (WidgetTester tester) async {
     final ScrollController scrollController = ScrollController();
@@ -797,5 +949,12 @@ void main() {
           color: const Color(0x80000000),
       ),
     );
-  });
+  },
+    variant: const TargetPlatformVariant(<TargetPlatform>{
+      TargetPlatform.linux,
+      TargetPlatform.macOS,
+      TargetPlatform.windows,
+      TargetPlatform.fuchsia,
+    }),
+  );
 }