Apply carriedVelocity unless substantially different (#79382)

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

@@ -274,6 +274,14 @@ class ScrollDragController implements Drag {
   static const Duration momentumRetainStationaryDurationThreshold =
       Duration(milliseconds: 20);
 
+  /// The minimum amount of velocity needed to apply the [carriedVelocity] at
+  /// the end of a drag. Expressed as a factor. For example with a
+  /// [carriedVelocity] of 2000, we will need a velocity of at least 1000 to
+  /// apply the [carriedVelocity] as well. If the velocity does not meet the
+  /// threshold the the [carriedVelocity] is lost. Decided by fair eyeballing
+  /// with the scroll_overlay platform test.
+  static const double momentumRetainVelocityThresholdFactor = 0.5;
+
   /// Maximum amount of time interval the drag can have consecutive stationary
   /// pointer update events before needing to break the
   /// [motionStartDistanceThreshold] to start motion again.
@@ -390,9 +398,17 @@ class ScrollDragController implements Drag {
       velocity = -velocity;
     _lastDetails = details;
 
+    if (_retainMomentum) {
       // Build momentum only if dragging in the same direction.
-    if (_retainMomentum && velocity.sign == carriedVelocity!.sign)
+      final bool isFlingingInSameDirection = velocity.sign == carriedVelocity!.sign;
+      // Build momentum only if the velocity of the last drag was not
+      // substantially lower than the carried momentum.
+      final bool isVelocityNotSubstantiallyLessThanCarriedMomentum =
+        velocity.abs() > carriedVelocity!.abs() * momentumRetainVelocityThresholdFactor;
+      if(isFlingingInSameDirection && isVelocityNotSubstantiallyLessThanCarriedMomentum) {
         velocity += carriedVelocity!;
+      }
+    }
     delegate.goBallistic(velocity);
   }
 

packages/flutter/test/widgets/scrollable_test.dart

@@ -178,6 +178,23 @@ void main() {
     expect(getScrollVelocity(tester), moreOrLessEquals(1000.0));
   });
 
+  testWidgets('A slower final fling does not apply carried momentum', (WidgetTester tester) async {
+    await pumpTest(tester, debugDefaultTargetPlatformOverride);
+    await tester.fling(find.byType(Scrollable), const Offset(0.0, -dragOffset), 1000.0);
+    await tester.pump(); // trigger fling
+    await tester.pump(const Duration(milliseconds: 10));
+    // Repeat the exact same motion to build momentum.
+    await tester.fling(find.byType(Scrollable), const Offset(0.0, -dragOffset), 1000.0);
+    await tester.pump(); // trigger the second fling
+    await tester.pump(const Duration(milliseconds: 10));
+    // Make a final fling that is much slower.
+    await tester.fling(find.byType(Scrollable), const Offset(0.0, -dragOffset), 200.0);
+    await tester.pump(); // trigger the third fling
+    await tester.pump(const Duration(milliseconds: 10));
+    // expect that there is no carried velocity
+    expect(getScrollVelocity(tester), lessThan(200.0));
+  }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
+
   testWidgets('No iOS/macOS momentum build with flings in opposite directions', (WidgetTester tester) async {
     await pumpTest(tester, debugDefaultTargetPlatformOverride);
     await tester.fling(find.byType(Scrollable), const Offset(0.0, -dragOffset), 1000.0);