Fix ScrollPosition.isScrollingNotifier.value for pointer scrolling (#113972)

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

@@ -810,8 +810,6 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   ///
   /// This method is very similar to [jumpTo], but [pointerScroll] will
   /// update the [ScrollDirection].
-  ///
-  // TODO(YeungKC): Support trackpad scroll, https://github.com/flutter/flutter/issues/23604.
   void pointerScroll(double delta);
 
   /// Calls [jumpTo] if duration is null or [Duration.zero], otherwise

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

@@ -222,8 +222,10 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
           -delta > 0.0 ? ScrollDirection.forward : ScrollDirection.reverse,
       );
       final double oldPixels = pixels;
-      forcePixels(targetPixels);
+      // Set the notifier before calling force pixels.
+      // This is set to false again after going ballistic below.
       isScrollingNotifier.value = true;
+      forcePixels(targetPixels);
       didStartScroll();
       didUpdateScrollPositionBy(pixels - oldPixels);
       didEndScroll();

packages/flutter/test/widgets/scroll_controller_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui' as ui;
+
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -362,4 +364,33 @@ void main() {
     expect(tester.getTopLeft(find.widgetWithText(SizedBox, 'Item 1')), Offset.zero);
 
   });
+
+  testWidgets('isScrollingNotifier works with pointer scroll', (WidgetTester tester) async {
+    Widget buildFrame(ScrollController controller) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: ListView(
+          controller: controller,
+          children: List<Widget>.generate(50, (int index) {
+            return SizedBox(height: 100.0, child: Text('Item $index'));
+          }).toList(),
+        ),
+      );
+    }
+
+    bool isScrolling = false;
+    final ScrollController controller = ScrollController();
+    controller.addListener((){
+      isScrolling = controller.position.isScrollingNotifier.value;
+    });
+    await tester.pumpWidget(buildFrame(controller));
+    final Offset scrollEventLocation = tester.getCenter(find.byType(ListView));
+    final TestPointer testPointer = TestPointer(1, ui.PointerDeviceKind.mouse);
+    // Create a hover event so that |testPointer| has a location when generating the scroll.
+    testPointer.hover(scrollEventLocation);
+    await tester.sendEventToBinding(testPointer.scroll(const Offset(0.0, 300.0)));
+    // When the listener was notified, the value of the isScrollingNotifier
+    // should have been true
+    expect(isScrolling, isTrue);
+  });
 }