Composited fixed-pos element should still cause repaints if it paintsIntoCompositedAncestor

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 86999cefa96bb1351365bcd0049c27d9d2c61733..2f8c349d0fd613e0a2d2944d37ca26ac4926cab9 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -807,6 +807,11 @@ bool ScrollingCoordinator::hasVisibleSlowRepaintViewportConstrainedObjects(Frame
         // Any explicit reason that a fixed position element is not composited shouldn't cause slow scrolling.
         if (!layer->isComposited() && layer->viewportConstrainedNotCompositedReason() == RenderLayer::NoNotCompositedReason)
             return true;
+
+        // Composited layers that actually paint into their enclosing ancestor
+        // must also force main thread scrolling.
+        if (layer->isComposited() && layer->backing()->paintsIntoCompositedAncestor())
+            return true;
     }
     return false;
 }