Avoid compositing fixed-position elements if they cannot scroll

Source/core/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 1949 matching lines @@
     }
 
     // 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. Start by
+    // assuming we can opt-out (i.e. no scrollable ancestor), and refine the answer below.
+    bool hasScrollableAncestor = false;
+
+    // The FrameView has the scrollbars associated with the top level viewport, so we have to
+    // check the FrameView in addition to the hierarchy of ancestors.
+    FrameView* frameView = m_renderView->frameView();
+    if (frameView && frameView->isScrollable())
+        hasScrollableAncestor = true;

[enne (OOO), 2013/05/02 01:04:07]

Thanks, this is way clearer.

+
+    RenderLayer* ancestor = layer->parent();
+    while (ancestor && !hasScrollableAncestor) {
+        if (frameView->containsScrollableArea(ancestor))
+            hasScrollableAncestor = true;
+        if (ancestor->renderer() == m_renderView)
+            break;
+        ancestor = ancestor->parent();
+    }
+
+    if (!hasScrollableAncestor) {
+        if (viewportConstrainedNotCompositedReason)
+            *viewportConstrainedNotCompositedReason = RenderLayer::NotCompositedForUnscrollableAncestors;
+        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 779 matching lines @@
     info.addMember(m_layerForScrollCorner, "layerForScrollCorner");
 #if ENABLE(RUBBER_BANDING)
     info.addMember(m_layerForOverhangAreas, "layerForOverhangAreas");
     info.addMember(m_contentShadowLayer, "contentShadowLayer");
     info.addMember(m_layerForTopOverhangArea, "layerForTopOverhangArea");
     info.addMember(m_layerForBottomOverhangArea, "layerForBottomOverhangArea");
 #endif
 }
 
 } // namespace WebCore