Fix scrollbar dragging into overscroll when not allowed (#90634)

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

@@ -16,6 +16,7 @@ import 'gesture_detector.dart';
 import 'media_query.dart';
 import 'notification_listener.dart';
 import 'primary_scroll_controller.dart';
+import 'scroll_configuration.dart';
 import 'scroll_controller.dart';
 import 'scroll_metrics.dart';
 import 'scroll_notification.dart';
@@ -1367,7 +1368,24 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     if (scrollOffsetGlobal != position.pixels) {
       // Ensure we don't drag into overscroll if the physics do not allow it.
       final double physicsAdjustment = position.physics.applyBoundaryConditions(position, scrollOffsetGlobal);
-      position.jumpTo(scrollOffsetGlobal - physicsAdjustment);
+      double newPosition = scrollOffsetGlobal - physicsAdjustment;
+
+      // The physics may allow overscroll when actually *scrolling*, but
+      // dragging on the scrollbar does not always allow us to enter overscroll.
+      switch(ScrollConfiguration.of(context).getPlatform(context)) {
+        case TargetPlatform.fuchsia:
+        case TargetPlatform.linux:
+        case TargetPlatform.macOS:
+        case TargetPlatform.windows:
+          newPosition = newPosition.clamp(0.0, position.maxScrollExtent);
+          break;
+        case TargetPlatform.iOS:
+        case TargetPlatform.android:
+          // We can only drag the scrollbar into overscroll on mobile
+          // platforms, and only if the physics allow it.
+          break;
+      }
+      position.jumpTo(newPosition);
     }
   }
 

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1025,6 +1025,139 @@ void main() {
     );
   });
 
+  testWidgets('Scrollbar thumb cannot be dragged into overscroll if the platform does not allow it', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: ScrollConfiguration(
+            // Don't apply a scrollbar automatically for this test.
+            behavior: const ScrollBehavior().copyWith(
+              scrollbars: false,
+              physics: const AlwaysScrollableScrollPhysics(),
+            ),
+            child: PrimaryScrollController(
+              controller: scrollController,
+              child: RawScrollbar(
+                isAlwaysShown: true,
+                controller: scrollController,
+                child: const SingleChildScrollView(
+                  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, 0.0, 800.0, 90.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+
+    // Try to drag the thumb into overscroll.
+    const double scrollAmount = -10.0;
+    final TestGesture dragScrollbarGesture = await tester.startGesture(const Offset(797.0, 45.0));
+    await tester.pumpAndSettle();
+    await dragScrollbarGesture.moveBy(const Offset(0.0, scrollAmount));
+    await tester.pumpAndSettle();
+
+    // The platform drag handling should not have allowed us to enter overscroll.
+    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, 0.0, 800.0, 90.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+
+    await dragScrollbarGesture.up();
+    await tester.pumpAndSettle();
+  }, variant: const TargetPlatformVariant(<TargetPlatform>{
+    TargetPlatform.macOS,
+    TargetPlatform.linux,
+    TargetPlatform.windows,
+    TargetPlatform.fuchsia,
+  }));
+
+  testWidgets('Scrollbar thumb can be dragged into overscroll if the platform allows it', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: ScrollConfiguration(
+            // Don't apply a scrollbar automatically for this test.
+            behavior: const ScrollBehavior().copyWith(
+              scrollbars: false,
+              physics: const AlwaysScrollableScrollPhysics(),
+            ),
+            child: PrimaryScrollController(
+              controller: scrollController,
+              child: RawScrollbar(
+                isAlwaysShown: true,
+                controller: scrollController,
+                child: const SingleChildScrollView(
+                  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, 0.0, 800.0, 90.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+
+    // Try to drag the thumb into overscroll.
+    const double scrollAmount = -10.0;
+    final TestGesture dragScrollbarGesture = await tester.startGesture(const Offset(797.0, 45.0));
+    await tester.pumpAndSettle();
+    await dragScrollbarGesture.moveBy(const Offset(0.0, scrollAmount));
+    await tester.pumpAndSettle();
+
+    // The platform drag handling should have allowed us to enter overscroll.
+    expect(scrollController.offset, lessThan(-66.0));
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          // The size of the scrollbar thumb shrinks when overscrolling
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 80.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+
+    await dragScrollbarGesture.up();
+    await tester.pumpAndSettle();
+  }, variant: const TargetPlatformVariant(<TargetPlatform>{
+    TargetPlatform.android,
+    TargetPlatform.iOS,
+  }));
+
   // Regression test for https://github.com/flutter/flutter/issues/66444
   testWidgets("RawScrollbar doesn't show when scroll the inner scrollable widget", (WidgetTester tester) async {
     final GlobalKey key1 = GlobalKey();