Add test for scrollbar.1.dart (flutter#151463)

Contributes to flutter#130459

It adds a test for
- `examples/api/lib/material/scrollbar/scrollbar.1.dart`

Author: ValentinVignal

dev/bots/check_code_samples.dart

@@ -322,7 +322,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/flexible_space_bar/flexible_space_bar.0_test.dart',
   'examples/api/test/material/chip/deletable_chip_attributes.on_deleted.0_test.dart',
   'examples/api/test/material/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0_test.dart',
-  'examples/api/test/material/scrollbar/scrollbar.1_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.0_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.1_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.2_test.dart',

examples/api/test/material/scrollbar/scrollbar.0_test.dart

@@ -18,4 +18,22 @@ void main() {
 
     expect(tester.takeException(), isNull);
   }, variant: TargetPlatformVariant.all());
+
+  testWidgets('The scrollbar should be painted when the user scrolls', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ScrollbarExampleApp(),
+    );
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 10)); // Wait for the thumb to start appearing.
+
+    expect(find.text('item 0'), findsOne);
+    expect(find.text('item 9'), findsNothing);
+    expect(find.byType(Scrollbar), isNot(paints..rect()));
+
+    await tester.fling(find.byType(Scrollbar).last, const Offset(0, -300), 10.0);
+
+    expect(find.text('item 0'), findsNothing);
+    expect(find.text('item 9'), findsOne);
+    expect(find.byType(Scrollbar).last, paints..rect());
+  });
 }

examples/api/test/material/scrollbar/scrollbar.1_test.dart

@@ -0,0 +1,29 @@
+// 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/material.dart';
+import 'package:flutter_api_samples/material/scrollbar/scrollbar.1.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('The scrollbar thumb should be visible at all time', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ScrollbarExampleApp(),
+    );
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 10)); // Wait for the thumb to start appearing.
+
+    expect(find.widgetWithText(AppBar, 'Scrollbar Sample'), findsOne);
+
+    expect(find.text('item 0'), findsOne);
+    expect(find.text('item 9'), findsNothing);
+    expect(find.byType(Scrollbar), paints..rect());
+
+    await tester.fling(find.byType(Scrollbar).last, const Offset(0, -300), 10.0);
+
+    expect(find.text('item 0'), findsNothing);
+    expect(find.text('item 9'), findsOne);
+    expect(find.byType(Scrollbar), paints..rect());
+  });
+}