Fix scrollbar drag gestures for reversed scrollables (#82764)

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

@@ -951,7 +951,7 @@ class RawScrollbar extends StatefulWidget {
 /// Provides defaults gestures for dragging the scrollbar thumb and tapping on the
 /// scrollbar track.
 class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProviderStateMixin<T> {
-  double? _dragScrollbarAxisPosition;
+  Offset? _dragScrollbarAxisOffset;
   ScrollController? _currentController;
   Timer? _fadeoutTimer;
   late AnimationController _fadeoutAnimationController;
@@ -1133,9 +1133,25 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     }
   }
 
-  void _updateScrollPosition(double primaryDelta) {
+  void _updateScrollPosition(Offset updatedOffset) {
     assert(_currentController != null);
+    assert(_dragScrollbarAxisOffset != null);
     final ScrollPosition position = _currentController!.position;
+    late double primaryDelta;
+    switch (position.axisDirection) {
+      case AxisDirection.up:
+        primaryDelta = _dragScrollbarAxisOffset!.dy - updatedOffset.dy;
+        break;
+      case AxisDirection.right:
+        primaryDelta = updatedOffset.dx -_dragScrollbarAxisOffset!.dx;
+        break;
+      case AxisDirection.down:
+        primaryDelta = updatedOffset.dy -_dragScrollbarAxisOffset!.dy;
+        break;
+      case AxisDirection.left:
+        primaryDelta = _dragScrollbarAxisOffset!.dx - updatedOffset.dx;
+        break;
+    }
 
     // Convert primaryDelta, the amount that the scrollbar moved since the last
     // time _updateScrollPosition was called, into the coordinate space of the scroll
@@ -1159,8 +1175,8 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     }
   }
 
-  /// Returns the [Axis] of the child scroll view, or null if the current scroll
+  /// Returns the [Axis] of the child scroll view, or null if the
-  /// controller does not have any attached positions.
+  /// current scroll controller does not have any attached positions.
   @protected
   Axis? getScrollbarDirection() {
     assert(_currentController != null);
@@ -1194,14 +1210,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     }
     _fadeoutTimer?.cancel();
     _fadeoutAnimationController.forward();
-    switch (direction) {
+    _dragScrollbarAxisOffset = localPosition;
-      case Axis.vertical:
-        _dragScrollbarAxisPosition = localPosition.dy;
-        break;
-      case Axis.horizontal:
-        _dragScrollbarAxisPosition = localPosition.dx;
-        break;
-    }
   }
 
   /// Handler called when a currently active long press gesture moves.
@@ -1214,16 +1223,8 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     if (direction == null) {
       return;
     }
-    switch(direction) {
+    _updateScrollPosition(localPosition);
-      case Axis.vertical:
+    _dragScrollbarAxisOffset = localPosition;
-        _updateScrollPosition(localPosition.dy - _dragScrollbarAxisPosition!);
-        _dragScrollbarAxisPosition = localPosition.dy;
-        break;
-      case Axis.horizontal:
-        _updateScrollPosition(localPosition.dx - _dragScrollbarAxisPosition!);
-        _dragScrollbarAxisPosition = localPosition.dx;
-        break;
-    }
   }
 
   /// Handler called when a long press has ended.
@@ -1234,7 +1235,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     if (direction == null)
       return;
     _maybeStartFadeoutTimer();
-    _dragScrollbarAxisPosition = null;
+    _dragScrollbarAxisOffset = null;
     _currentController = null;
   }
 
@@ -1303,7 +1304,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
       _fadeoutTimer?.cancel();
       scrollbarPainter.update(notification.metrics, notification.metrics.axisDirection);
     } else if (notification is ScrollEndNotification) {
-      if (_dragScrollbarAxisPosition == null)
+      if (_dragScrollbarAxisOffset == null)
         _maybeStartFadeoutTimer();
     }
     return false;

packages/flutter/test/cupertino/scrollbar_test.dart

@@ -167,6 +167,80 @@ void main() {
     await tester.pump(_kScrollbarFadeDuration);
   });
 
+  testWidgets('Scrollbar thumb can be dragged with long press - reverse', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: const CupertinoScrollbar(
+              child: SingleChildScrollView(
+                reverse: true,
+                child: SizedBox(width: 4000.0, height: 4000.0),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(scrollController.offset, 0.0);
+
+    // Scroll a bit.
+    const double scrollAmount = 10.0;
+    final TestGesture scrollGesture = await tester.startGesture(tester.getCenter(find.byType(SingleChildScrollView)));
+    // Scroll up by swiping down.
+    await scrollGesture.moveBy(const Offset(0.0, scrollAmount));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+    // Scrollbar thumb is fully showing and scroll offset has moved by
+    // scrollAmount.
+    expect(find.byType(CupertinoScrollbar), paints..rrect(
+      color: _kScrollbarColor.color,
+    ));
+    expect(scrollController.offset, scrollAmount);
+    await scrollGesture.up();
+    await tester.pump();
+
+    int hapticFeedbackCalls = 0;
+    SystemChannels.platform.setMockMethodCallHandler((MethodCall methodCall) async {
+      if (methodCall.method == 'HapticFeedback.vibrate') {
+        hapticFeedbackCalls++;
+      }
+    });
+
+    // Long press on the scrollbar thumb and expect a vibration after it resizes.
+    expect(hapticFeedbackCalls, 0);
+    final TestGesture dragScrollbarGesture = await tester.startGesture(const Offset(796.0, 550.0));
+    await tester.pump(_kLongPressDuration);
+    expect(hapticFeedbackCalls, 0);
+    await tester.pump(_kScrollbarResizeDuration);
+    // Allow the haptic feedback some slack.
+    await tester.pump(const Duration(milliseconds: 1));
+    expect(hapticFeedbackCalls, 1);
+
+    // Drag the thumb up to scroll up.
+    await dragScrollbarGesture.moveBy(const Offset(0.0, -scrollAmount));
+    await tester.pump(const Duration(milliseconds: 100));
+    await dragScrollbarGesture.up();
+    await tester.pumpAndSettle();
+
+    // The view has scrolled more than it would have by a swipe gesture of the
+    // same distance.
+    expect(scrollController.offset, greaterThan(scrollAmount * 2));
+    // The scrollbar thumb is still fully visible.
+    expect(find.byType(CupertinoScrollbar), paints..rrect(
+      color: _kScrollbarColor.color,
+    ));
+
+    // Let the thumb fade out so all timers have resolved.
+    await tester.pump(_kScrollbarTimeToFade);
+    await tester.pump(_kScrollbarFadeDuration);
+  });
+
   testWidgets('Scrollbar changes thickness and radius when dragged', (WidgetTester tester) async {
     const double thickness = 20;
     const double thicknessWhileDragging = 40;
@@ -727,6 +801,80 @@ void main() {
     await tester.pump(_kScrollbarFadeDuration);
   });
 
+  testWidgets('Scrollbar thumb can be dragged with long press - horizontal axis, reverse', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: CupertinoScrollbar(
+            controller: scrollController,
+            child: SingleChildScrollView(
+              reverse: true,
+              controller: scrollController,
+              scrollDirection: Axis.horizontal,
+              child: const SizedBox(width: 4000.0, height: 4000.0),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(scrollController.offset, 0.0);
+
+    // Scroll a bit.
+    const double scrollAmount = 10.0;
+    final TestGesture scrollGesture = await tester.startGesture(tester.getCenter(find.byType(SingleChildScrollView)));
+    // Scroll right by swiping right.
+    await scrollGesture.moveBy(const Offset(scrollAmount, 0.0));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+    // Scrollbar thumb is fully showing and scroll offset has moved by
+    // scrollAmount.
+    expect(find.byType(CupertinoScrollbar), paints..rrect(
+      color: _kScrollbarColor.color,
+    ));
+    expect(scrollController.offset, scrollAmount);
+    await scrollGesture.up();
+    await tester.pump();
+
+    int hapticFeedbackCalls = 0;
+    SystemChannels.platform.setMockMethodCallHandler((MethodCall methodCall) async {
+      if (methodCall.method == 'HapticFeedback.vibrate') {
+        hapticFeedbackCalls++;
+      }
+    });
+
+    // Long press on the scrollbar thumb and expect a vibration after it resizes.
+    expect(hapticFeedbackCalls, 0);
+    final TestGesture dragScrollbarGesture = await tester.startGesture(const Offset(750.0, 596.0));
+    await tester.pump(_kLongPressDuration);
+    expect(hapticFeedbackCalls, 0);
+    await tester.pump(_kScrollbarResizeDuration);
+    // Allow the haptic feedback some slack.
+    await tester.pump(const Duration(milliseconds: 1));
+    expect(hapticFeedbackCalls, 1);
+
+    // Drag the thumb to scroll back to the right.
+    await dragScrollbarGesture.moveBy(const Offset(-scrollAmount, 0.0));
+    await tester.pump(const Duration(milliseconds: 100));
+    await dragScrollbarGesture.up();
+    await tester.pumpAndSettle();
+
+    // The view has scrolled more than it would have by a swipe gesture of the
+    // same distance.
+    expect(scrollController.offset, greaterThan(scrollAmount * 2));
+    // The scrollbar thumb is still fully visible.
+    expect(find.byType(CupertinoScrollbar), paints..rrect(
+      color: _kScrollbarColor.color,
+    ));
+
+    // Let the thumb fade out so all timers have resolved.
+    await tester.pump(_kScrollbarTimeToFade);
+    await tester.pump(_kScrollbarFadeDuration);
+  });
+
   testWidgets('Tapping the track area pages the Scroll View', (WidgetTester tester) async {
     final ScrollController scrollController = ScrollController();
     await tester.pumpWidget(

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1339,6 +1339,61 @@ void main() {
     );
   });
 
+  testWidgets('Scrollbar thumb can be dragged in reverse', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: RawScrollbar(
+              isAlwaysShown: true,
+              controller: scrollController,
+              child: const SingleChildScrollView(
+                reverse: true,
+                child: SizedBox(width: 4000.0, height: 4000.0),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 510.0, 800.0, 600.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+
+    // Drag the thumb up to scroll up.
+    const double scrollAmount = 10.0;
+    final TestGesture dragScrollbarGesture = await tester.startGesture(const Offset(797.0, 550.0));
+    await tester.pumpAndSettle();
+    await dragScrollbarGesture.moveBy(const Offset(0.0, -scrollAmount));
+    await tester.pumpAndSettle();
+    await dragScrollbarGesture.up();
+    await tester.pumpAndSettle();
+
+    // The view has scrolled more than it would have by a swipe gesture of the
+    // same distance.
+    expect(scrollController.offset, greaterThan(scrollAmount * 2));
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 500.0, 800.0, 590.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+  });
   testWidgets('ScrollbarPainter asserts if scrollbarOrientation is used with wrong axisDirection', (WidgetTester tester) async {
     final ScrollbarPainter painter = ScrollbarPainter(
       color: _kScrollbarColor,