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

Source/core/page/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 1336 matching lines @@
     const bool isCompositedContentLayer = contentsInCompositedLayer();
 
     // Get the rects of the fixed objects visible in the rectToScroll
     Region regionToUpdate;
     ViewportConstrainedObjectSet::const_iterator end = m_viewportConstrainedObjects->end();
     for (ViewportConstrainedObjectSet::const_iterator it = m_viewportConstrainedObjects->begin(); it != end; ++it) {
         RenderObject* renderer = *it;
         if (!renderer->style()->hasViewportConstrainedPosition())
             continue;
 
-        if (renderer->isComposited())
-            continue;
-
         // Fixed items should always have layers.
         ASSERT(renderer->hasLayer());
         RenderLayer* layer = toRenderBoxModelObject(renderer)->layer();
 
+        // Composited layers may still actually paint into their ancestor.
+        // If that happens, the viewport constrained object needs to be
+        // repainted on scroll.
+        if (layer->isComposited() && !layer->backing()->paintsIntoCompositedAncestor())
+            continue;
+
         if (layer->viewportConstrainedNotCompositedReason() == RenderLayer::NotCompositedForBoundsOutOfView
             || layer->viewportConstrainedNotCompositedReason() == RenderLayer::NotCompositedForNoVisibleContent) {
             // Don't invalidate for invisible fixed layers.
             continue;
         }
 
         if (layer->hasAncestorWithFilterOutsets()) {
             // If the fixed layer has a blur/drop-shadow filter applied on at least one of its parents, we cannot
             // scroll using the fast path, otherwise the outsets of the filter will be moved around the page.
             return false;
@@ skipping 2032 matching lines @@
         return frame().document()->existingAXObjectCache();
     return 0;
 }
 
 bool FrameView::isMainFrame() const
 {
     return m_frame->page() && m_frame->page()->mainFrame() == m_frame;
 }
 
 } // namespace WebCore