Avoid compositing fixed-position elements if they cannot scroll

Source/WebCore/rendering/RenderLayerCompositor.cpp

 /*
  * Copyright (C) 2009, 2010 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 1958 matching lines @@
         return false;
     }
 
     // Don't promote fixed position elements that are descendants of a non-view container, e.g. transformed elements.
     // They will stay fixed wrt the container rather than the enclosing frame.
     if (container != m_renderView) {
         if (viewportConstrainedNotCompositedReason)
             *viewportConstrainedNotCompositedReason = RenderLayer::NotCompositedForNonViewContainer;
         return false;
     }
-    
+
+    // If the fixed-position element does not have any scrollable ancestor between it and its container,
+    // then we do not need to spend compositor resources for it.
+    if (container->isRenderView()) {
+
+        RenderLayer* ancestor = layer->parent();
+        bool noScrollableAncestor = true;
+
+        if (m_renderView->frameView() && (m_renderView->frameView()->isScrollable()))
+            noScrollableAncestor = false;

[trchen, 2013/04/10 01:24:03]

I'm concerned about the platforms that use impl-side pinch zoom (Android &
CrOS). FrameView::isScrollable becomes a dynamic property. In that case I think
we should always create composited layers for RenderView-contained
fixed-position elements.

+
+        while (ancestor && noScrollableAncestor) {
+            if (ancestor->hasHorizontalScrollbar() || ancestor->hasVerticalScrollbar())
+                noScrollableAncestor = false;
+            ancestor = ancestor->parent();
+        }
+
+        if (noScrollableAncestor)
+            return false;
+    }
+
     // Subsequent tests depend on layout. If we can't tell now, just keep things the way they are until layout is done.
     if (!m_inPostLayoutUpdate) {
         m_reevaluateCompositingAfterLayout = true;
         return layer->isComposited();
     }
 
     bool paintsContent = layer->isVisuallyNonEmpty() || layer->hasVisibleDescendant();
     if (!paintsContent) {
         if (viewportConstrainedNotCompositedReason)
             *viewportConstrainedNotCompositedReason = RenderLayer::NotCompositedForNoVisibleContent;
@@ skipping 851 matching lines @@
     info.addMember(m_layerForOverhangAreas, "layerForOverhangAreas");
     info.addMember(m_contentShadowLayer, "contentShadowLayer");
     info.addMember(m_layerForTopOverhangArea, "layerForTopOverhangArea");
     info.addMember(m_layerForBottomOverhangArea, "layerForBottomOverhangArea");
     info.addMember(m_layerForHeader, "layerForHeader");
     info.addMember(m_layerForFooter, "layerForFooter");
 #endif
 }
 
 } // namespace WebCore