Add an intermediate function to generate 2 paint-order lists.

Source/core/rendering/RenderLayer.h

 /*
  * Copyright (C) 2003, 2009, 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 795 matching lines @@
         NotCompositedForBoundsOutOfView,
         NotCompositedForNonViewContainer,
         NotCompositedForNoVisibleContent,
     };
 
     void setViewportConstrainedNotCompositedReason(ViewportConstrainedNotCompositedReason reason) { m_compositingProperties.viewportConstrainedNotCompositedReason = reason; }
     ViewportConstrainedNotCompositedReason viewportConstrainedNotCompositedReason() const { return static_cast<ViewportConstrainedNotCompositedReason>(m_compositingProperties.viewportConstrainedNotCompositedReason); }
 
     bool isOutOfFlowRenderFlowThread() const { return renderer()->isOutOfFlowRenderFlowThread(); }
 
+#ifndef NDEBUG
+    String paintOrderListsAsText();
+#endif
+    enum PaintOrderListType {BeforePromote, AfterPromote};
+    PassRefPtr<NodeList> paintOrderList(PaintOrderListType type);
+
 private:
     enum CollectLayersBehavior { StopAtStackingContexts, StopAtStackingContainers };
 
     void updateZOrderLists();
     void rebuildZOrderLists();
-    void rebuildZOrderLists(CollectLayersBehavior, OwnPtr<Vector<RenderLayer*> >&, OwnPtr<Vector<RenderLayer*> >&);
+    // See the comment for collectLayers for information about the
+    // layerToForceAsStackingContainer parameter.

[Julien - ping for review, 2013/04/24 15:44:01]

Nit: You don't need to wrap at 80 columns in Blink, this would be totally fine
on the previous line.

[hartmanng, 2013/04/24 17:02:41]

Done.

+    void rebuildZOrderLists(CollectLayersBehavior, OwnPtr<Vector<RenderLayer*> >&, OwnPtr<Vector<RenderLayer*> >&, const RenderLayer* layerToForceAsStackingContainer = 0);
     void clearZOrderLists();
 
     void updateNormalFlowList();
 
     bool isStackingContext(const RenderStyle* style) const { return !style->hasAutoZIndex() || isRootLayer(); }
 
     bool isDirtyStackingContainer() const { return m_zOrderListsDirty && isStackingContainer(); }
 
     void setAncestorChainHasSelfPaintingLayerDescendant();
     void dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
 
     bool acceleratedCompositingForOverflowScrollEnabled() const;
     void updateDescendantsAreContiguousInStackingOrder();
     void updateDescendantsAreContiguousInStackingOrderRecursive(const HashMap<const RenderLayer*, int>&, int& minIndex, int& maxIndex, int& count, bool firstIteration);
+    void collectBeforePromotionZOrderList(RenderLayer* ancestorStackingContext, OwnPtr<Vector<RenderLayer*> >& posZOrderListBeforePromote, OwnPtr<Vector<RenderLayer*> >& negZOrderListBeforePromote, size_t& posZOrderListSizeBeforePromote, size_t& negZOrderListSizeBeforePromote);
+    void collectAfterPromotionZOrderList(RenderLayer* ancestorStackingContext, OwnPtr<Vector<RenderLayer*> >& posZOrderListAfterPromote, OwnPtr<Vector<RenderLayer*> >& negZOrderListAfterPromote, size_t& posZOrderListSizeAfterPromote, size_t& negZOrderListSizeAfterPromote);
 
     void computeRepaintRects(const RenderLayerModelObject* repaintContainer, const RenderGeometryMap* = 0);
     void computeRepaintRectsIncludingDescendants();
     void clearRepaintRects();
 
     void clipToRect(RenderLayer* rootLayer, GraphicsContext*, const LayoutRect& paintDirtyRect, const ClipRect&,
                     BorderRadiusClippingRule = IncludeSelfForBorderRadius);
     void restoreClip(GraphicsContext*, const LayoutRect& paintDirtyRect, const ClipRect&);
 
     bool shouldRepaintAfterLayout() const;
@@ skipping 36 matching lines @@
     void* operator new(size_t) throw();
 
     void setNextSibling(RenderLayer* next) { m_next = next; }
     void setPreviousSibling(RenderLayer* prev) { m_previous = prev; }
     void setParent(RenderLayer* parent);
     void setFirstChild(RenderLayer* first) { m_first = first; }
     void setLastChild(RenderLayer* last) { m_last = last; }
 
     LayoutPoint renderBoxLocation() const { return renderer()->isBox() ? toRenderBox(renderer())->location() : LayoutPoint(); }
 
-    void collectLayers(bool includeHiddenLayers, CollectLayersBehavior, OwnPtr<Vector<RenderLayer*> >&, OwnPtr<Vector<RenderLayer*> >&);
+    // layerToForceAsStackingContainer allows us to build pre-promotion and
+    // post-promotion layer lists, by allowing us to treat a layer as if it is a
+    // stacking context, without adding a new member to RenderLayer or modifying
+    // the style (which could cause extra allocations).
+    void collectLayers(bool includeHiddenLayers, CollectLayersBehavior, OwnPtr<Vector<RenderLayer*> >&, OwnPtr<Vector<RenderLayer*> >&, const RenderLayer* layerToForceAsStackingContainer = 0);
 
     struct LayerPaintingInfo {
         LayerPaintingInfo(RenderLayer* inRootLayer, const LayoutRect& inDirtyRect, PaintBehavior inPaintBehavior, const LayoutSize& inSubPixelAccumulation, RenderObject* inPaintingRoot = 0, RenderRegion*inRegion = 0, OverlapTestRequestMap* inOverlapTestRequests = 0)
             : rootLayer(inRootLayer)
             , paintingRoot(inPaintingRoot)
             , paintDirtyRect(inDirtyRect)
             , subPixelAccumulation(inSubPixelAccumulation)
             , region(inRegion)
             , overlapTestRequests(inOverlapTestRequests)
             , paintBehavior(inPaintBehavior)
@@ skipping 400 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const WebCore::RenderLayer*);
 void showLayerTree(const WebCore::RenderObject*);
 #endif
 
 #endif // RenderLayer_h