Non-composited, fixed position should trigger main thread scrolling

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 868 matching lines @@
 }
 #endif
 
 bool ScrollingCoordinator::hasVisibleSlowRepaintViewportConstrainedObjects(FrameView* frameView) const
 {
     const FrameView::ViewportConstrainedObjectSet* viewportConstrainedObjects = frameView->viewportConstrainedObjects();
     if (!viewportConstrainedObjects)
         return false;
 
     for (FrameView::ViewportConstrainedObjectSet::const_iterator it = viewportConstrainedObjects->begin(), end = viewportConstrainedObjects->end(); it != end; ++it) {
-        RenderObject* viewportConstrainedObject = *it;
-        if (!viewportConstrainedObject->isBoxModelObject() || !viewportConstrainedObject->hasLayer())
-            return true;
-        RenderLayer* layer = toRenderBoxModelObject(viewportConstrainedObject)->layer();
-        // Composited layers that actually paint into their enclosing ancestor
-        // must also force main thread scrolling.
-        if (layer->compositingState() == HasOwnBackingButPaintsIntoAncestor)
+        RenderObject* renderer = *it;
+        ASSERT(renderer->isBoxModelObject() && renderer->hasLayer());
+        ASSERT(renderer->style()->position() == FixedPosition);
+        RenderLayer* layer = toRenderBoxModelObject(renderer)->layer();
+
+        // Whether the RenderLayer scrolls with the viewport is a tree-depenent
+        // property and our viewportConstrainedObjects collection is maintained
+        // with only RenderObject-level information.
+        if (!layer->scrollsWithViewport())
+            continue;
+
+        // We're only smart enough to scroll viewport-constrainted objects
+        // in the compositor if they have their own backing or they paint
+        // into a grouped back (which necessarily all have the same viewport
+        // constraints).
+        CompositingState compositingState = layer->compositingState();
+        if (compositingState != PaintsIntoOwnBacking && compositingState != PaintsIntoGroupedBacking)
             return true;
     }
     return false;
 }
 
 MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() const
 {
     // The main thread scrolling reasons are applicable to scrolls of the main
     // frame. If it does not exist or if it is not scrollable, there is no
     // reason to force main thread scrolling.
@@ skipping 41 matching lines @@
     bool frameIsScrollable = frameView && frameView->isScrollable();
     if (frameIsScrollable != m_wasFrameScrollable)
         return true;
 
     if (WebLayer* scrollLayer = frameView ? toWebLayer(frameView->layerForScrolling()) : 0)
         return blink::WebSize(frameView->contentsSize()) != scrollLayer->bounds();
     return false;
 }
 
 } // namespace blink