Add test for raw_scrollbar.2.dart (#158161)

Contributes to https://github.com/flutter/flutter/issues/130459

It adds a test for
- `examples/api/lib/widgets/scrollbar/raw_scrollbar.2.dart`

dev/bots/check_code_samples.dart

@@ -312,6 +312,5 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/color_scheme/dynamic_content_color.0_test.dart',
   'examples/api/test/widgets/image/image.frame_builder.0_test.dart',
   'examples/api/test/widgets/image/image.loading_builder.0_test.dart',
-  'examples/api/test/widgets/scrollbar/raw_scrollbar.2_test.dart',
   'examples/api/test/widgets/scrollbar/raw_scrollbar.desktop.0_test.dart',
 };

examples/api/lib/widgets/scrollbar/raw_scrollbar.0.dart

@@ -34,6 +34,12 @@ class RawScrollbarExample extends StatefulWidget {
 class _RawScrollbarExampleState extends State<RawScrollbarExample> {
   final ScrollController _firstController = ScrollController();
 
+  @override
+  void dispose() {
+    _firstController.dispose();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     return LayoutBuilder(builder: (BuildContext context, BoxConstraints constraints) {

examples/api/lib/widgets/scrollbar/raw_scrollbar.2.dart

@@ -32,6 +32,12 @@ class RawScrollbarExample extends StatefulWidget {
 class _RawScrollbarExampleState extends State<RawScrollbarExample> {
   final ScrollController _controllerOne = ScrollController();
 
+  @override
+  void dispose() {
+    _controllerOne.dispose();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     return RawScrollbar(

examples/api/lib/widgets/scrollbar/raw_scrollbar.desktop.0.dart

@@ -34,6 +34,12 @@ class DesktopExample extends StatefulWidget {
 class _DesktopExampleState extends State<DesktopExample> {
   final ScrollController controller = ScrollController();
 
+  @override
+  void dispose() {
+    controller.dispose();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     return LayoutBuilder(builder: (BuildContext context, BoxConstraints constraints) {

examples/api/test/widgets/scrollbar/raw_scrollbar.2_test.dart

@@ -0,0 +1,37 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/scrollbar/raw_scrollbar.2.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('The scrollbar remains visible', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.RawScrollbarExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'RawScrollbar Sample'), findsOne);
+    expect(find.byType(RawScrollbar), findsOne);
+    expect(find.byType(GridView), findsOne);
+
+    expect(find.byType(RawScrollbar), paints..clipRect());
+
+    expect(find.text('item 0'), findsOne);
+    expect(find.text('item 1'), findsOne);
+    expect(find.text('item 2'), findsOne);
+
+    final TestPointer pointer = TestPointer(1, PointerDeviceKind.mouse);
+    pointer.hover(tester.getCenter(find.byType(GridView).first));
+    await tester.sendEventToBinding(pointer.scroll(const Offset(0.0, 1000)));
+    await tester.pumpAndSettle();
+
+    expect(find.text('item 15'), findsOne);
+    expect(find.text('item 16'), findsOne);
+    expect(find.text('item 17'), findsOne);
+
+    expect(find.byType(RawScrollbar), paints..clipRect());
+  });
+}