Fix: Assertion failure in RawScrollbarState with dynamic controller assignment (#173156)

Fix: Assertion failure in RawScrollbarState with dynamic controller
assignment
fixes: #172614 

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

@@ -1456,6 +1456,14 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     } else if (_effectiveScrollController != null && enableGestures) {
       // Interactive scrollbars need to be properly configured. If it is visible
       // for interaction, ensure we are set up properly.
+      // Don't assert immediately if we're in the middle of updating the widget
+      // as the controller may not be attached yet in that frame.
+      if (_fadeoutAnimationController.status == AnimationStatus.forward &&
+          (widget.thumbVisibility ?? false)) {
+        // When thumbVisibility is true and we're animating forward,
+        // the check is already scheduled by _debugScheduleCheckHasValidScrollPosition.
+        return;
+      }
       assert(_debugCheckHasValidScrollPosition());
     }
   }

packages/flutter/test/widgets/scrollbar_test.dart

@@ -3635,4 +3635,79 @@ The provided ScrollController cannot be shared by multiple ScrollView widgets.''
     expect(size.height - rtlRect1.bottom, 4);
     expect(rtlRect1.left, 3);
   });
+
+  testWidgets(
+    'RawScrollbar does not assert when dynamically assigning controller with thumbVisibility true',
+    (WidgetTester tester) async {
+      // This test reproduces the issue from https://github.com/flutter/flutter/issues/172614
+      // where dynamically assigning a controller and setting thumbVisibility to true
+      // in the same frame caused an assertion failure.
+
+      final ScrollController scrollController = ScrollController();
+      addTearDown(scrollController.dispose);
+
+      bool useController = false;
+      bool thumbVisible = false;
+      bool interactive = false;
+
+      await tester.pumpWidget(
+        StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return Directionality(
+              textDirection: TextDirection.ltr,
+              child: MediaQuery(
+                data: const MediaQueryData(),
+                child: Column(
+                  children: <Widget>[
+                    ElevatedButton(
+                      onPressed: () {
+                        setState(() {
+                          // This reproduces the exact scenario from the linked issue:
+                          // Initially the controller is null, thumbVisibility is false, and interactive is false.
+                          // Then, in the next frame, the controller is non-null, thumbVisibility is true, and interactive is true.
+                          useController = true;
+                          thumbVisible = true;
+                          interactive = true;
+                        });
+                      },
+                      child: const Text('Enable Scrollbar'),
+                    ),
+                    Expanded(
+                      child: RawScrollbar(
+                        controller: useController ? scrollController : null,
+                        thumbVisibility: thumbVisible,
+                        interactive: interactive,
+                        child: SingleChildScrollView(
+                          controller: useController ? scrollController : null,
+                          child: const SizedBox(height: 2000.0, width: 100.0),
+                        ),
+                      ),
+                    ),
+                  ],
+                ),
+              ),
+            );
+          },
+        ),
+      );
+
+      await tester.pumpAndSettle();
+      expect(find.byType(RawScrollbar), findsOneWidget);
+
+      await tester.tap(find.text('Enable Scrollbar'));
+      await tester.pumpAndSettle();
+
+      // Verify scrollbar is now working properly and doesn't throw an assertion.
+      final Finder scrollbarFinder = find.byType(RawScrollbar);
+      expect(scrollbarFinder, findsOneWidget);
+      expect(scrollController.offset, 0.0);
+
+      // Test that scrolling works without assertions.
+      await tester.drag(find.byType(SingleChildScrollView), const Offset(0.0, -100.0));
+      await tester.pumpAndSettle();
+
+      // Verify the scroll happened.
+      expect(scrollController.offset, greaterThan(0.0));
+    },
+  );
 }