Update RawScrollbar to support the track (#97335)

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

@@ -142,7 +142,7 @@ class Scrollbar extends StatelessWidget {
   )
   final bool? isAlwaysShown;
 
-  /// Controls the track visibility.
+  /// {@macro flutter.widgets.Scrollbar.trackVisibility}
   ///
   /// If this property is null, then [ScrollbarThemeData.trackVisibility] of
   /// [ThemeData.scrollbarTheme] is used. If that is also null, the default value
@@ -246,7 +246,7 @@ class _MaterialScrollbar extends RawScrollbar {
     required Widget child,
     ScrollController? controller,
     bool? thumbVisibility,
-    this.trackVisibility,
+    bool? trackVisibility,
     this.showTrackOnHover,
     this.hoverThickness,
     double? thickness,
@@ -261,6 +261,7 @@ class _MaterialScrollbar extends RawScrollbar {
          thumbVisibility: thumbVisibility,
          thickness: thickness,
          radius: radius,
+         trackVisibility: trackVisibility,
          fadeDuration: _kScrollbarFadeDuration,
          timeToFade: _kScrollbarTimeToFade,
          pressDuration: Duration.zero,
@@ -269,7 +270,6 @@ class _MaterialScrollbar extends RawScrollbar {
          scrollbarOrientation: scrollbarOrientation,
        );
 
-  final bool? trackVisibility;
   final bool? showTrackOnHover;
   final double? hoverThickness;
 
@@ -350,7 +350,7 @@ class _MaterialScrollbarState extends RawScrollbarState<_MaterialScrollbar> {
     final Color onSurface = _colorScheme.onSurface;
     final Brightness brightness = _colorScheme.brightness;
     return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
-      if (_trackVisibility.resolve(states)) {
+      if (showScrollbar && _trackVisibility.resolve(states)) {
         return _scrollbarTheme.trackColor?.resolve(states)
           ?? (brightness == Brightness.light
             ? onSurface.withOpacity(0.03)
@@ -364,7 +364,7 @@ class _MaterialScrollbarState extends RawScrollbarState<_MaterialScrollbar> {
     final Color onSurface = _colorScheme.onSurface;
     final Brightness brightness = _colorScheme.brightness;
     return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
-      if (_trackVisibility.resolve(states)) {
+      if (showScrollbar && _trackVisibility.resolve(states)) {
         return _scrollbarTheme.trackBorderColor?.resolve(states)
           ?? (brightness == Brightness.light
             ? onSurface.withOpacity(0.1)

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

@@ -875,6 +875,9 @@ class RawScrollbar extends StatefulWidget {
     this.thumbColor,
     this.minThumbLength = _kMinThumbExtent,
     this.minOverscrollLength,
+    this.trackVisibility,
+    this.trackColor,
+    this.trackBorderColor,
     this.fadeDuration = _kScrollbarFadeDuration,
     this.timeToFade = _kScrollbarTimeToFade,
     this.pressDuration = Duration.zero,
@@ -894,6 +897,10 @@ class RawScrollbar extends StatefulWidget {
          'Scrollbar thumb appearance should only be controlled with thumbVisibility, '
          'isAlwaysShown is deprecated.'
        ),
+       assert(
+         !((thumbVisibility == false || isAlwaysShown == false) && trackVisibility == true),
+         'A scrollbar track cannot be drawn without a scrollbar thumb.'
+       ),
        assert(minThumbLength != null),
        assert(minThumbLength >= 0),
        assert(minOverscrollLength == null || minOverscrollLength <= minThumbLength),
@@ -1199,6 +1206,36 @@ class RawScrollbar extends StatefulWidget {
   /// When null, it will default to the value of [minThumbLength].
   final double? minOverscrollLength;
 
+  /// {@template flutter.widgets.Scrollbar.trackVisibility}
+  /// Indicates that the scrollbar track should be visible.
+  ///
+  /// When true, the scrollbar track will always be visible so long as the thumb
+  /// is visible. If the scrollbar thumb is not visible, the track will not be
+  /// visible either.
+  ///
+  /// Defaults to false when null.
+  /// {@endtemplate}
+  ///
+  /// Subclass [Scrollbar] can hide and show the scrollbar thumb in response to
+  /// [MaterialState]s by using [ScrollbarThemeData.trackVisibility].
+  final bool? trackVisibility;
+
+  /// The color of the scrollbar track.
+  ///
+  /// The scrollbar track will only be visible when [trackVisibility] and
+  /// [thumbVisibility] are true.
+  ///
+  /// If null, defaults to Color(0x08000000).
+  final Color? trackColor;
+
+  /// The color of the scrollbar track's border.
+  ///
+  /// The scrollbar track will only be visible when [trackVisibility] and
+  /// [thumbVisibility] are true.
+  ///
+  /// If null, defaults to Color(0x1a000000).
+  final Color? trackBorderColor;
+
   /// The [Duration] of the fade animation.
   ///
   /// Cannot be null, defaults to a [Duration] of 300 milliseconds.
@@ -1304,6 +1341,8 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @protected
   bool get showScrollbar => widget.isAlwaysShown ?? widget.thumbVisibility ?? false;
 
+  bool get _showTrack => showScrollbar && (widget.trackVisibility ?? false);
+
   /// Overridable getter to indicate is gestures should be enabled on the
   /// scrollbar.
   ///
@@ -1470,6 +1509,8 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   void updateScrollbarPainter() {
     scrollbarPainter
       ..color = widget.thumbColor ?? const Color(0x66BCBCBC)
+      ..trackColor = _showTrack ? const Color(0x08000000) : const Color(0x00000000)
+      ..trackBorderColor = _showTrack ? const Color(0x1a000000) : const Color(0x00000000)
       ..textDirection = Directionality.of(context)
       ..thickness = widget.thickness ?? _kScrollbarThickness
       ..radius = widget.radius

packages/flutter/test/widgets/scrollbar_test.dart

@@ -2245,4 +2245,71 @@ void main() {
     expect(painter.shouldRepaint(createPainter(minOverscrollLength: 1.0)), true);
     expect(painter.shouldRepaint(createPainter(scrollbarOrientation: ScrollbarOrientation.bottom)), true);
   });
+
+  testWidgets('Scrollbar track can be drawn', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: RawScrollbar(
+              thumbVisibility: true,
+              trackVisibility: true,
+              controller: scrollController,
+              child: const SingleChildScrollView(
+                child: SizedBox(width: 4000.0, height: 4000.0),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0),
+          color: const Color(0x08000000),
+        )
+        ..line(
+          p1: const Offset(794.0, 0.0),
+          p2: const Offset(794.0, 600.0),
+          strokeWidth: 1.0,
+          color: const Color(0x1a000000),
+        )
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 90.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+  });
+
+  testWidgets('Scrollbar asserts that a visible track has a visible thumb', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    Widget _buildApp() {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: RawScrollbar(
+              thumbVisibility: false,
+              trackVisibility: true,
+              controller: scrollController,
+              child: const SingleChildScrollView(
+                child: SizedBox(width: 4000.0, height: 4000.0),
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+    expect(() => tester.pumpWidget(_buildApp()), throwsAssertionError);
+  });
 }