Prevent Scrollbar axis flipping when there is an oriented scroll controller (#87698)

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

@@ -1533,15 +1533,36 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
     );
   }
 
+  // ScrollController takes precedence over ScrollNotification
+  bool _shouldUpdatePainter(Axis notificationAxis) {
+    final ScrollController? scrollController = widget.controller ??
+        PrimaryScrollController.of(context);
+    // Only update the painter of this scrollbar if the notification
+    // metrics do not conflict with the information we have from the scroll
+    // controller.
+    return
+      // We do not have a scroll controller dictating axis.
+      scrollController == null
+      // The scroll controller is not attached to a position.
+      || !scrollController.hasClients
+      // The notification matches the scroll controller's axis.
+      || scrollController.position.axis == notificationAxis;
+  }
+
   bool _handleScrollMetricsNotification(ScrollMetricsNotification notification) {
     if (!widget.notificationPredicate(ScrollUpdateNotification(metrics: notification.metrics, context: notification.context)))
       return false;
+
     if (showScrollbar) {
       if (_fadeoutAnimationController.status != AnimationStatus.forward
           && _fadeoutAnimationController.status != AnimationStatus.completed)
         _fadeoutAnimationController.forward();
     }
-    scrollbarPainter.update(notification.metrics, notification.metrics.axisDirection);
+
+    final ScrollMetrics metrics = notification.metrics;
+    if (_shouldUpdatePainter(metrics.axis)) {
+      scrollbarPainter.update(metrics, metrics.axisDirection);
+    }
     return false;
   }
 
@@ -1555,7 +1576,10 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
       if (_fadeoutAnimationController.status != AnimationStatus.dismissed
           && _fadeoutAnimationController.status != AnimationStatus.reverse)
         _fadeoutAnimationController.reverse();
-      scrollbarPainter.update(metrics, metrics.axisDirection);
+
+      if (_shouldUpdatePainter(metrics.axis)) {
+        scrollbarPainter.update(metrics, metrics.axisDirection);
+      }
       return false;
     }
 
@@ -1567,7 +1591,10 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
         _fadeoutAnimationController.forward();
 
       _fadeoutTimer?.cancel();
-      scrollbarPainter.update(notification.metrics, notification.metrics.axisDirection);
+
+      if (_shouldUpdatePainter(metrics.axis)) {
+        scrollbarPainter.update(metrics, metrics.axisDirection);
+      }
     } else if (notification is ScrollEndNotification) {
       if (_dragScrollbarAxisOffset == null)
         _maybeStartFadeoutTimer();

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1637,4 +1637,65 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.byType(RawScrollbar), isNot(paints..rect())); // Not shown.
   });
+
+  testWidgets('Scrollbar will not flip axes based on notification is there is a scroll controller', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/87697
+    final ScrollController verticalScrollController = ScrollController();
+    final ScrollController horizontalScrollController = ScrollController();
+    Widget buildFrame() {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: RawScrollbar(
+            isAlwaysShown: true,
+            controller: verticalScrollController,
+            // This scrollbar will receive scroll notifications from both nested
+            // scroll views of opposite axes, but should stay on the vertical
+            // axis that its scroll controller is associated with.
+            notificationPredicate: (ScrollNotification notification) => notification.depth <= 1,
+            child: SingleChildScrollView(
+              controller: verticalScrollController,
+              child: SingleChildScrollView(
+                scrollDirection: Axis.horizontal,
+                controller: horizontalScrollController,
+                child: const SizedBox(
+                  width: 1000.0,
+                  height: 1000.0,
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame());
+    await tester.pumpAndSettle();
+    expect(verticalScrollController.offset, 0.0);
+    expect(horizontalScrollController.offset, 0.0);
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 360.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+
+    // Move the horizontal scroll view. The vertical scrollbar should not flip.
+    horizontalScrollController.jumpTo(10.0);
+    expect(verticalScrollController.offset, 0.0);
+    expect(horizontalScrollController.offset, 10.0);
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 360.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+  });
 }