Rework composited scrolling updates.

Source/WebCore/rendering/RenderLayerCompositor.cpp

Index: Source/WebCore/rendering/RenderLayerCompositor.cpp
diff --git a/Source/WebCore/rendering/RenderLayerCompositor.cpp b/Source/WebCore/rendering/RenderLayerCompositor.cpp
index d49df157b71d16d3dfb4c9338d8e85178bce5db7..65c35e245cd4d5b9b777de4bebee187dbf5d7b66 100644
--- a/Source/WebCore/rendering/RenderLayerCompositor.cpp
+++ b/Source/WebCore/rendering/RenderLayerCompositor.cpp
@@ -323,6 +323,22 @@ bool RenderLayerCompositor::hasAnyAdditionalCompositedLayers(const RenderLayer*
     return m_compositedLayerCount > (rootLayer->isComposited() ? 1 : 0);
 }
 
+void RenderLayerCompositor::updateDescendantsAreContiguousInStackingOrderRecursive(RenderLayer* layer)
+{
+    for (RenderLayer* child = layer->firstChild(); child; child = child->nextSibling())
+        updateDescendantsAreContiguousInStackingOrderRecursive(child);
+
+    if (layer->isStackingContext() && layer->m_descendantsAreContiguousInStackingOrderDirty)
+        layer->updateDescendantsAreContiguousInStackingOrder();
+}
+
+void RenderLayerCompositor::updateNeedsCompositedScrolling(RenderLayer* layer)
+{
+    layer->updateNeedsCompositedScrolling();
+    for (RenderLayer* child = layer->firstChild(); child; child = child->nextSibling())
+        updateNeedsCompositedScrolling(child);
+}
+
 void RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType updateType, RenderLayer* updateRoot)
 {
     m_updateCompositingLayersTimer.stop();
@@ -383,6 +399,11 @@ void RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType update
 #endif
 
     if (checkForHierarchyUpdate) {
+        // FIXME(vollick): can we skip this altogether in some cases?
+        updateRoot->updateDescendantDependentFlags();
+        updateDescendantsAreContiguousInStackingOrderRecursive(updateRoot);
+        updateNeedsCompositedScrolling(updateRoot);
+
         // Go through the layers in presentation order, so that we can compute which RenderLayers need compositing layers.
         // FIXME: we could maybe do this and the hierarchy udpate in one pass, but the parenting logic would be more complex.
         CompositingState compState(updateRoot, m_compositingConsultsOverlap);