Added scrollbar example (#75761)

packages/flutter/lib/src/material/scrollbar.dart

@@ -34,7 +34,54 @@ const Duration _kScrollbarTimeToFade = Duration(milliseconds: 600);
 /// [showTrackOnHover]. The thickness of the track and scrollbar thumb will
 /// become larger when hovering, unless overridden by [hoverThickness].
 ///
-// TODO(Piinks): Add code sample
+/// {@tool dartpad --template=stateless_widget_scaffold}
+/// This sample shows a [Scrollbar] that executes a fade animation as scrolling occurs.
+/// The Scrollbar will fade into view as the user scrolls, and fade out when scrolling stops.
+/// ```dart
+/// Widget build(BuildContext context) {
+///   return Scrollbar(
+///     child: GridView.builder(
+///       itemCount: 120,
+///       gridDelegate:
+///         const SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 3),
+///       itemBuilder: (BuildContext context, int index) {
+///         return Center(
+///           child: Text('item $index'),
+///         );
+///       },
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
+/// {@tool dartpad --template=stateful_widget_scaffold}
+/// When isAlwaysShown is true, the scrollbar thumb will remain visible without the
+/// fade animation. This requires that a ScrollController is provided to controller,
+/// or that the PrimaryScrollController is available.
+/// ```dart
+/// final ScrollController _controllerOne = ScrollController();
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return Scrollbar(
+///     controller: _controllerOne,
+///     isAlwaysShown: true,
+///     child: GridView.builder(
+///       controller: _controllerOne,
+///       itemCount: 120,
+///       gridDelegate:
+///         const SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 3),
+///       itemBuilder: (BuildContext context, int index) {
+///         return Center(
+///           child: Text('item $index'),
+///         );
+///       },
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
 ///
 /// See also:
 ///