Remove Scrollbar.isAlwaysShown assert based on Scrollbar.controller (#72316)

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

@@ -561,10 +561,11 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
 /// visible.
 ///
 /// By default, the thumb will fade in and out as the child scroll view
-/// scrolls. When [isAlwaysShown] is true, and a [controller] is specified, the
-/// scrollbar thumb will remain visible without the fade animation.
+/// scrolls. 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.
 ///
-/// Scrollbars are interactive and will use the [PrimaryScrollController] if
+/// Scrollbars are interactive and will also use the [PrimaryScrollController] if
 /// a [controller] is not set. Scrollbar thumbs can be dragged along the main axis
 /// of the [ScrollView] to change the [ScrollPosition]. Tapping along the track
 /// exclusive of the thumb will trigger a [ScrollIncrementType.page] based on
@@ -606,10 +607,7 @@ class RawScrollbar extends StatefulWidget {
     this.fadeDuration = _kScrollbarFadeDuration,
     this.timeToFade = _kScrollbarTimeToFade,
     this.pressDuration = Duration.zero,
-  }) : assert(
-         !isAlwaysShown || controller != null,
-         'When isAlwaysShown is true, a ScrollController must be provided.',
-       ),
+  }) : assert(isAlwaysShown != null),
        assert(child != null),
        assert(fadeDuration != null),
        assert(timeToFade != null),
@@ -681,8 +679,9 @@ class RawScrollbar extends StatefulWidget {
   /// When false, the scrollbar will be shown during scrolling
   /// and will fade out otherwise.
   ///
-  /// When true, the scrollbar will always be visible and never fade out. The
-  /// [controller] property must be set in this case.
+  /// When true, the scrollbar will always be visible and never fade out. If the
+  /// [controller] property has not been set, the [PrimaryScrollController] will
+  /// be used.
   ///
   /// Defaults to false.
   ///
@@ -826,7 +825,13 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
         // Wait one frame and cause an empty scroll event.  This allows the
         // thumb to show immediately when isAlwaysShown is true. A scroll
         // event is required in order to paint the thumb.
-        widget.controller!.position.didUpdateScrollPositionBy(0);
+        final ScrollController? scrollController = widget.controller ?? PrimaryScrollController.of(context);
+        assert(
+          scrollController != null,
+          'A ScrollController is required when Scrollbar.isAlwaysShown is true. '
+          'Either Scrollbar.controller was not provided, or a PrimaryScrollController could not be found.',
+        );
+        scrollController!.position.didUpdateScrollPositionBy(0);
       }
     });
   }

packages/flutter/test/cupertino/scrollbar_test.dart

@@ -263,16 +263,16 @@ void main() {
     await tester.pump(_kScrollbarFadeDuration);
   });
 
-  testWidgets('When isAlwaysShown is true, must pass a controller',
+  testWidgets('When isAlwaysShown is true, must pass a controller or find PrimaryScrollController',
       (WidgetTester tester) async {
     Widget viewWithScroll() {
-      return Directionality(
+      return const Directionality(
         textDirection: TextDirection.ltr,
         child: MediaQuery(
-          data: const MediaQueryData(),
+          data: MediaQueryData(),
           child: CupertinoScrollbar(
             isAlwaysShown: true,
-            child: const SingleChildScrollView(
+            child: SingleChildScrollView(
               child: SizedBox(
                 width: 4000.0,
                 height: 4000.0,
@@ -283,12 +283,12 @@ void main() {
       );
     }
 
-    expect(() async {
-      await tester.pumpWidget(viewWithScroll());
-    }, throwsAssertionError);
+    await tester.pumpWidget(viewWithScroll());
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
   });
 
-  testWidgets('When isAlwaysShown is true, must pass a controller that is attached to a scroll view',
+  testWidgets('When isAlwaysShown is true, must pass a controller or find PrimarySCrollController that is attached to a scroll view',
       (WidgetTester tester) async {
     final ScrollController controller = ScrollController();
     Widget viewWithScroll() {
@@ -315,6 +315,39 @@ void main() {
     expect(exception, isAssertionError);
   });
 
+  testWidgets('On first render with isAlwaysShown: true, the thumb shows with PrimaryScrollController', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    Widget viewWithScroll() {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: controller,
+            child: Builder(
+              builder: (BuildContext context) {
+                return const CupertinoScrollbar(
+                  isAlwaysShown: true,
+                  child: SingleChildScrollView(
+                    primary: true,
+                    child: SizedBox(
+                      width: 4000.0,
+                      height: 4000.0,
+                    ),
+                  ),
+                );
+              },
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(viewWithScroll());
+    await tester.pumpAndSettle();
+    expect(find.byType(CupertinoScrollbar), paints..rect());
+  });
+
   testWidgets('On first render with isAlwaysShown: true, the thumb shows',
       (WidgetTester tester) async {
     final ScrollController controller = ScrollController();

packages/flutter/test/material/scrollbar_test.dart

@@ -122,15 +122,15 @@ void main() {
     expect(canvas.invocations.isEmpty, isTrue);
   });
 
-  testWidgets('When isAlwaysShown is true, must pass a controller',
+  testWidgets('When isAlwaysShown is true, must pass a controller or find PrimaryScrollController',
       (WidgetTester tester) async {
     Widget viewWithScroll() {
       return _buildBoilerplate(
         child: Theme(
           data: ThemeData(),
-          child: Scrollbar(
+          child: const Scrollbar(
             isAlwaysShown: true,
-            child: const SingleChildScrollView(
+            child: SingleChildScrollView(
               child: SizedBox(
                 width: 4000.0,
                 height: 4000.0,
@@ -141,12 +141,12 @@ void main() {
       );
     }
 
-    expect(() async {
-      await tester.pumpWidget(viewWithScroll());
-    }, throwsAssertionError);
+    await tester.pumpWidget(viewWithScroll());
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
   });
 
-  testWidgets('When isAlwaysShown is true, must pass a controller that is attached to a scroll view',
+  testWidgets('When isAlwaysShown is true, must pass a controller that is attached to a scroll view or find PrimaryScrollController',
       (WidgetTester tester) async {
     final ScrollController controller = ScrollController();
     Widget viewWithScroll() {
@@ -199,6 +199,38 @@ void main() {
     expect(find.byType(Scrollbar), paints..rect());
   });
 
+  testWidgets('On first render with isAlwaysShown: true, the thumb shows with PrimaryScrollController', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    Widget viewWithScroll() {
+      return _buildBoilerplate(
+        child: Theme(
+          data: ThemeData(),
+          child: PrimaryScrollController(
+            controller: controller,
+            child: Builder(
+              builder: (BuildContext context) {
+                return const Scrollbar(
+                  isAlwaysShown: true,
+                  child: SingleChildScrollView(
+                    primary: true,
+                    child: SizedBox(
+                      width: 4000.0,
+                      height: 4000.0,
+                    ),
+                  ),
+                );
+              },
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(viewWithScroll());
+    await tester.pumpAndSettle();
+    expect(find.byType(Scrollbar), paints..rect());
+  });
+
   testWidgets('On first render with isAlwaysShown: false, the thumb is hidden',
       (WidgetTester tester) async {
     final ScrollController controller = ScrollController();