Fix scroll jump when NestedScrollPosition is inertia-cancelled. (#116689)

* Fix scroll jump when NestedScrollPosition is inertia-cancelled.

* Switch to using pointerScroll(0)

Author: moffatman

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

@@ -903,7 +903,10 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
     // If an update is made to pointer scrolling here, consider if the same
     // (or similar) change should be made in
     // ScrollPositionWithSingleContext.pointerScroll.
-    assert(delta != 0.0);
+    if (delta == 0.0) {
+      goBallistic(0.0);
+      return;
+    }
 
     goIdle();
     updateUserScrollDirection(

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

@@ -212,7 +212,10 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     // If an update is made to pointer scrolling here, consider if the same
     // (or similar) change should be made in
     // _NestedScrollCoordinator.pointerScroll.
-    assert(delta != 0.0);
+    if (delta == 0.0) {
+      goBallistic(0.0);
+      return;
+    }
 
     final double targetPixels =
         math.min(math.max(pixels + delta, minScrollExtent), maxScrollExtent);

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

@@ -780,7 +780,7 @@ class ScrollableState extends State<Scrollable> with TickerProviderStateMixin, R
         GestureBinding.instance.pointerSignalResolver.register(event, _handlePointerScroll);
       }
     } else if (event is PointerScrollInertiaCancelEvent) {
-      position.jumpTo(position.pixels);
+      position.pointerScroll(0);
       // Don't use the pointer signal resolver, all hit-tested scrollables should stop.
     }
   }

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -1012,6 +1012,54 @@ void main() {
       );
     });
 
+    testWidgets('Inertia-cancel event does not modify either position.', (WidgetTester tester) async {
+      final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
+      await tester.pumpWidget(buildTest(
+        key: globalKey,
+        expanded: false,
+      ));
+
+      double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      expect(appBarHeight, 104.0);
+      final double scrollExtent = appBarHeight + 50.0;
+      expect(globalKey.currentState!.outerController.offset, 0.0);
+      expect(globalKey.currentState!.innerController.offset, 0.0);
+
+      // The scroll gesture should occur in the inner body, so the whole
+      // scroll view is scrolled.
+      final TestGesture gesture = await tester.startGesture(Offset(
+        0.0,
+        appBarHeight + 1.0,
+      ));
+      await gesture.moveBy(Offset(0.0, -scrollExtent));
+      await tester.pump();
+
+      appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      // This is not an expanded AppBar.
+      expect(appBarHeight, 104.0);
+      // The outer scroll controller should show an offset of the applied
+      // scrollExtent.
+      expect(globalKey.currentState!.outerController.offset, appBarHeight);
+      // the inner scroll controller should have scrolled equivalent to the
+      // difference between the applied scrollExtent and the outer extent.
+      expect(
+        globalKey.currentState!.innerController.offset,
+        scrollExtent - appBarHeight,
+      );
+
+      final TestPointer testPointer = TestPointer(3, ui.PointerDeviceKind.trackpad);
+      await tester.sendEventToBinding(testPointer.addPointer(
+        location: Offset(0.0, appBarHeight + 1.0)
+      ));
+      await tester.sendEventToBinding(testPointer.scrollInertiaCancel());
+      // ensure no change.
+      expect(globalKey.currentState!.outerController.offset, appBarHeight);
+      expect(
+        globalKey.currentState!.innerController.offset,
+        scrollExtent - appBarHeight,
+      );
+    });
+
     testWidgets('scrolling by less than the expanded outer extent does not scroll the inner body', (WidgetTester tester) async {
       final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
       await tester.pumpWidget(buildTest(key: globalKey));