Use ScrollbarTheme instead Theme for Scrollbar (#113237)

Author: Oleh-Sv

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

@@ -399,7 +399,7 @@ class _MaterialScrollbarState extends RawScrollbarState<_MaterialScrollbar> {
   void didChangeDependencies() {
     final ThemeData theme = Theme.of(context);
     _colorScheme = theme.colorScheme;
-    _scrollbarTheme = theme.scrollbarTheme;
+    _scrollbarTheme = ScrollbarTheme.of(context);
     switch (theme.platform) {
       case TargetPlatform.android:
         _useAndroidScrollbar = true;

packages/flutter/test/material/scrollbar_theme_test.dart

@@ -206,6 +206,50 @@ void main() {
     }),
   );
 
+  testWidgets(
+    'Scrollbar uses values from ScrollbarTheme if exists instead of values from Theme',
+    (WidgetTester tester) async {
+      final ScrollbarThemeData scrollbarTheme = _scrollbarTheme();
+      final ScrollController scrollController = ScrollController();
+      await tester.pumpWidget(
+        MaterialApp(
+          theme: ThemeData(
+            scrollbarTheme: scrollbarTheme,
+          ),
+          home: ScrollConfiguration(
+            behavior: const NoScrollbarBehavior(),
+            child: ScrollbarTheme(
+              data: _scrollbarTheme().copyWith(
+                thumbColor: MaterialStateProperty.all(const Color(0xFF000000)),
+              ),
+              child: Scrollbar(
+                thumbVisibility: true,
+                controller: scrollController,
+                child: SingleChildScrollView(
+                  controller: scrollController,
+                  child: const SizedBox(width: 4000.0, height: 4000.0),
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+      // Idle scrollbar behavior
+      expect(
+        find.byType(Scrollbar),
+        paints
+          ..rrect(
+            rrect: RRect.fromRectAndRadius(
+              const Rect.fromLTRB(785.0, 10.0, 795.0, 97.0),
+              const Radius.circular(6.0),
+            ),
+            color: const Color(0xFF000000),
+          ),
+      );
+    },
+  );
+
   testWidgets('ScrollbarTheme can disable gestures', (WidgetTester tester) async {
     final ScrollController scrollController = ScrollController();
     await tester.pumpWidget(MaterialApp(