Defer the real work in updateCompositingLayers until it's really needed.

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 684 matching lines @@
         child->updateLayerPositionsAfterScroll(geometryMap, flags);
 
     // We don't update our reflection as scrolling is a translation which does not change the size()
     // of an object, thus RenderReplica will still repaint itself properly as the layer position was
     // updated above.
 
     if (geometryMap)
         geometryMap->popMappingsToAncestor(parent());
 }
 
-void RenderLayer::positionNewlyCreatedOverflowControls()
-{
-    if (!compositedLayerMapping()->hasUnpositionedOverflowControlsLayers())
-        return;
-
-    RenderGeometryMap geometryMap(UseTransforms);
-    RenderView* view = renderer()->view();
-    if (this != view->layer() && parent())
-        geometryMap.pushMappingsToAncestor(parent(), 0);
-
-    LayoutPoint offsetFromRoot = LayoutPoint(geometryMap.absolutePoint(FloatPoint()));
-    positionOverflowControls(toIntSize(roundedIntPoint(offsetFromRoot)));
-}
-
 bool RenderLayer::hasBlendMode() const
 {
     return RuntimeEnabledFeatures::cssCompositingEnabled() && renderer()->hasBlendMode();
 }
 
 void RenderLayer::updateBlendMode()
 {
     if (!RuntimeEnabledFeatures::cssCompositingEnabled())
         return;
 
@@ skipping 1375 matching lines @@
     updateIsNormalFlowOnly();
     updateSelfPaintingLayer();
 
     if (isStackingContainer())
         dirtyZOrderLists();
     else
         clearZOrderLists();
 
     dirtyStackingContainerZOrderLists();
 
-    compositor()->setShouldReevaluateCompositingAfterLayout();
+    compositor()->setNeedsToRecomputeCompositingRequirements();
     compositor()->setCompositingLayersNeedRebuild();
 }
 
 static inline int adjustedScrollDelta(int beginningDelta) {
     // This implemention matches Firefox's.
     // http://mxr.mozilla.org/firefox/source/toolkit/content/widgets/browser.xml#856.
     const int speedReducer = 12;
 
     int adjustedDelta = beginningDelta / speedReducer;
     if (adjustedDelta > 1)
@@ skipping 2378 matching lines @@
     if (m_negZOrderList)
         m_negZOrderList->clear();
     m_zOrderListsDirty = true;
 
     m_descendantsAreContiguousInStackingOrderDirty = true;
 
     if (!renderer()->documentBeingDestroyed()) {
         compositor()->setNeedsUpdateCompositingRequirementsState();
         compositor()->setCompositingLayersNeedRebuild();
         if (acceleratedCompositingForOverflowScrollEnabled())
-            compositor()->setShouldReevaluateCompositingAfterLayout();
+            compositor()->setNeedsToRecomputeCompositingRequirements();
     }
 }
 
 void RenderLayer::dirtyStackingContainerZOrderLists()
 {
     // Any siblings in the ancestor stacking context could also be affected.
     // Changing z-index, for example, could cause us to stack in between a
     // sibling's descendants, meaning that we have to recompute
     // m_descendantsAreContiguousInStackingOrder for that sibling.
     dirtySiblingStackingContextCanBePromotedToStackingContainer();
@@ skipping 16 matching lines @@
 {
     ASSERT(m_layerListMutationAllowed);
 
     if (m_normalFlowList)
         m_normalFlowList->clear();
     m_normalFlowListDirty = true;
 
     if (!renderer()->documentBeingDestroyed()) {
         compositor()->setCompositingLayersNeedRebuild();
         if (acceleratedCompositingForOverflowScrollEnabled())
-            compositor()->setShouldReevaluateCompositingAfterLayout();
+            compositor()->setNeedsToRecomputeCompositingRequirements();
     }
 }
 
 void RenderLayer::rebuildZOrderLists()
 {
     ASSERT(m_layerListMutationAllowed);
     ASSERT(isDirtyStackingContainer());
     rebuildZOrderLists(m_posZOrderList, m_negZOrderList);
     m_zOrderListsDirty = false;
 }
@@ skipping 704 matching lines @@
 }
 
 IntRect RenderLayer::scrollCornerAndResizerRect() const
 {
     if (!m_scrollableArea)
         return IntRect();
 
     return m_scrollableArea->scrollCornerAndResizerRect();
 }
 
+void RenderLayer::positionOverflowControls()
+{
+    RenderGeometryMap geometryMap(UseTransforms);
+    RenderView* view = renderer()->view();
+    if (this != view->layer() && parent())
+        geometryMap.pushMappingsToAncestor(parent(), 0);
+
+    LayoutPoint offsetFromRoot = LayoutPoint(geometryMap.absolutePoint(FloatPoint()));
+    positionOverflowControls(toIntSize(roundedIntPoint(offsetFromRoot)));
+}
+
 void RenderLayer::positionOverflowControls(const IntSize& offsetFromRoot)
 {
     if (m_scrollableArea)
         m_scrollableArea->positionOverflowControls(offsetFromRoot);
 }
 
 void RenderLayer::paintScrollCorner(GraphicsContext* context, const IntPoint& paintOffset, const IntRect& damageRect)
 {
     if (m_scrollableArea)
         m_scrollableArea->paintScrollCorner(context, paintOffset, damageRect);
@@ skipping 41 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif