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

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index 675fbc718bef1643c67778a5c9ce81ad96af06ce..233099d119bb22b2766264f3772ff74fc94bffad 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -62,6 +62,7 @@ class FilterOperations;
 class HitTestRequest;
 class HitTestResult;
 class HitTestingTransformState;
+class PaintOrderLists;
 class RenderFlowThread;
 class RenderGeometryMap;
 class RenderLayerBacking;
@@ -836,12 +837,17 @@ public:
 
     bool isOutOfFlowRenderFlowThread() const { return renderer()->isOutOfFlowRenderFlowThread(); }
 
+    String paintOrderListsAsText();

[shawnsingh, 2013/04/18 02:22:14]

Should this be const?  Hopefully it's possible to do that...

[hartmanng, 2013/04/18 17:14:00]

Unfortunately it's not possible. We need to call
collectBeforeAfterPromotionZOrderLists(), which is necessarily non-const (due to
temporary changes of m_needsCompositedScrolling and m_isNormalFlowOnly).

+    PassRefPtr<PaintOrderLists> paintOrderLists();

[shawnsingh, 2013/04/18 02:22:14]

Also should this be const?  Hopefully it's possible to do that...

[hartmanng, 2013/04/18 17:14:00]

Same as above.

+
 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
+    // layerToForceAsStackingContext parameter.
+    void rebuildZOrderLists(CollectLayersBehavior, OwnPtr<Vector<RenderLayer*> >&, OwnPtr<Vector<RenderLayer*> >&, const RenderLayer* layerToForceAsStackingContext = 0);
     void clearZOrderLists();
 
     void updateNormalFlowList();
@@ -856,6 +862,11 @@ private:
     bool acceleratedCompositingForOverflowScrollEnabled() const;
     void updateDescendantsAreContiguousInStackingOrder();
     void updateDescendantsAreContiguousInStackingOrderRecursive(const HashMap<const RenderLayer*, int>&, int& minIndex, int& maxIndex, int& count, bool firstIteration);
+    void collectBeforeAfterPromotionZOrderLists(RenderLayer* ancestorStackingContext,
+        OwnPtr<Vector<RenderLayer*> >& posZOrderListBeforePromote,
+        OwnPtr<Vector<RenderLayer*> >& negZOrderListBeforePromote,
+        OwnPtr<Vector<RenderLayer*> >& posZOrderListAfterPromote,
+        OwnPtr<Vector<RenderLayer*> >& negZOrderListAfterPromote);
 
     void computeRepaintRects(const RenderLayerModelObject* repaintContainer, const RenderGeometryMap* = 0);
     void computeRepaintRectsIncludingDescendants();
@@ -912,7 +923,11 @@ private:
 
     LayoutPoint renderBoxLocation() const { return renderer()->isBox() ? toRenderBox(renderer())->location() : LayoutPoint(); }
 
-    void collectLayers(bool includeHiddenLayers, CollectLayersBehavior, OwnPtr<Vector<RenderLayer*> >&, OwnPtr<Vector<RenderLayer*> >&);
+    // layerToForceAsStackingContext 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* layerToForceAsStackingContext = 0);
 
     struct LayerPaintingInfo {
         LayerPaintingInfo(RenderLayer* inRootLayer, const LayoutRect& inDirtyRect, PaintBehavior inPaintBehavior, const LayoutSize& inSubPixelAccumulation, RenderObject* inPaintingRoot = 0, RenderRegion*inRegion = 0, OverlapTestRequestMap* inOverlapTestRequests = 0)