Stop CupertinoScrollbar's track from paging the scroll view on tap (#152197)

Fixes https://github.com/flutter/flutter/issues/120429

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

@@ -211,6 +211,14 @@ class _CupertinoScrollbarState extends RawScrollbarState<CupertinoScrollbar> {
     }
   }
 
+  @override
+  void handleTrackTapDown(TapDownDetails details) {
+    // On iOS, tapping the track does not page towards the position of the tap.
+    if (ScrollConfiguration.of(context).getPlatform(context) != TargetPlatform.iOS) {
+      super.handleTrackTapDown(details);
+    }
+  }
+
   @override
   void dispose() {
     _thicknessAnimationController.dispose();

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

@@ -1799,7 +1799,11 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     _cachedController = null;
   }
 
-  void _handleTrackTapDown(TapDownDetails details) {
+  /// Handler called when the track is tapped in order to page in the tapped
+  /// direction.
+  @protected
+  @mustCallSuper
+  void handleTrackTapDown(TapDownDetails details) {
     // The Scrollbar should page towards the position of the tap on the track.
     assert(_debugCheckHasValidScrollPosition());
     _cachedController = _effectiveScrollController;
@@ -2018,7 +2022,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
           customPaintKey: _scrollbarPainterKey,
         ),
         (_TrackTapGestureRecognizer instance) {
-          instance.onTapDown = _handleTrackTapDown;
+          instance.onTapDown = handleTrackTapDown;
         },
       );
 

packages/flutter/test/cupertino/scrollbar_test.dart

@@ -1017,7 +1017,7 @@ void main() {
     await tester.pump(kScrollbarFadeDuration);
   });
 
-  testWidgets('Tapping the track area pages the Scroll View', (WidgetTester tester) async {
+  testWidgets('Tapping the track area pages the Scroll View except on iOS', (WidgetTester tester) async {
     final ScrollController scrollController = ScrollController();
     addTearDown(scrollController.dispose);
     await tester.pumpWidget(
@@ -1075,7 +1075,64 @@ void main() {
         rrect: RRect.fromLTRBR(794.0, 3.0, 797.0, 359.4, const Radius.circular(1.5)),
       ),
     );
-  });
+  }, variant: TargetPlatformVariant.all(excluding: <TargetPlatform>{TargetPlatform.iOS}));
+
+  testWidgets('Tapping the track area does not page the Scroll View on iOS', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    addTearDown(scrollController.dispose);
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: CupertinoScrollbar(
+            thumbVisibility: true,
+            controller: scrollController,
+            child: SingleChildScrollView(
+              controller: scrollController,
+              child: const SizedBox(width: 1000.0, height: 1000.0),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+    expect(
+      find.byType(CupertinoScrollbar),
+      paints..rrect(
+        color: _kScrollbarColor.color,
+        rrect: RRect.fromLTRBR(794.0, 3.0, 797.0, 359.4, const Radius.circular(1.5)),
+      ),
+    );
+
+    // Tap on the track area below the thumb.
+    await tester.tapAt(const Offset(796.0, 550.0));
+    await tester.pumpAndSettle();
+
+    expect(scrollController.offset, 0.0);
+    expect(
+      find.byType(CupertinoScrollbar),
+      paints..rrect(
+        color: _kScrollbarColor.color,
+        rrect: RRect.fromLTRBR(794.0, 3.0, 797.0, 359.4, const Radius.circular(1.5)),
+      ),
+    );
+
+    // Tap on the track area above the thumb.
+    await tester.tapAt(const Offset(796.0, 50.0));
+    await tester.pumpAndSettle();
+
+    expect(scrollController.offset, 0.0);
+    expect(
+      find.byType(CupertinoScrollbar),
+      paints..rrect(
+        color: _kScrollbarColor.color,
+        rrect: RRect.fromLTRBR(794.0, 3.0, 797.0, 359.4, const Radius.circular(1.5)),
+      ),
+    );
+  }, variant: TargetPlatformVariant.only(TargetPlatform.iOS));
 
   testWidgets('Throw if interactive with the bar when no position attached', (WidgetTester tester) async {
     final ScrollController scrollController = ScrollController();