NOT FOR REVIEW - Update comp-scrolling state at a well defined point in the pipeline.

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index dffb6999ea46b40e65a624eb5517db9e241a1a2c..5efb48c6f0db303fa755dcf6d4c33d21b57de424 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -69,6 +69,7 @@
 #include "core/page/UseCounter.h"
 #include "core/page/animation/AnimationController.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
+#include "core/platform/chromium/TraceEvent.h"
 #include "core/platform/FloatConversion.h"
 #include "core/platform/HistogramSupport.h"
 #include "core/platform/PlatformMouseEvent.h"
@@ -113,6 +114,7 @@
 #include "core/rendering/svg/RenderSVGResourceClipper.h"
 #include <wtf/MemoryInstrumentationVector.h>
 #include <wtf/StdLibExtras.h>
+#include <wtf/TemporaryChange.h>
 #include <wtf/text/CString.h>
 #include <wtf/UnusedParam.h>
 
@@ -143,10 +145,11 @@ RenderLayer::RenderLayer(RenderLayerModelObject* renderer)
     , m_hasSelfPaintingLayerDescendant(false)
     , m_hasSelfPaintingLayerDescendantDirty(false)
     , m_hasOutOfFlowPositionedDescendant(false)
-    , m_hasOutOfFlowPositionedDescendantDirty(true)
+    , m_forceNeedsCompositedScrolling(DoNotForceCompositedScrolling)
     , m_needsCompositedScrolling(false)
-    , m_descendantsAreContiguousInStackingOrder(false)
-    , m_descendantsAreContiguousInStackingOrderDirty(true)
+    , m_updatingNeedsCompositedScrolling(false)
+    , m_canBePromotedToStackingContainer(false)
+    , m_updatingCanBePromotedToStackingContainer(false)
     , m_isRootLayer(renderer->isRenderView())
     , m_usedTransparency(false)
     , m_paintingInsideReflection(false)
@@ -482,173 +485,249 @@ bool RenderLayer::acceleratedCompositingForOverflowScrollEnabled() const
         && renderer()->frame()->page()->settings()->acceleratedCompositingForOverflowScrollEnabled();
 }
 
-// If we are a stacking container, then this function will determine if our
-// descendants for a contiguous block in stacking order. This is required in
-// order for an element to be safely promoted to a stacking container. It is safe
-// to become a stacking container if this change would not alter the stacking
-// order of layers on the page. That can only happen if a non-descendant appear
-// between us and our descendants in stacking order. Here's an example:
-//
-//                                 this
-//                                /  |  \.
-//                               A   B   C
-//                              /\   |   /\.
-//                             0 -8  D  2  7
-//                                   |
-//                                   5
-//
-// I've labeled our normal flow descendants A, B, C, and D, our stacking
-// container descendants with their z indices, and us with 'this' (we're a
-// stacking container and our zIndex doesn't matter here). These nodes appear in
-// three lists: posZOrder, negZOrder, and normal flow (keep in mind that normal
-// flow layers don't overlap). So if we arrange these lists in order we get our
-// stacking order:
-//
-//                     [-8], [A-D], [0, 2, 5, 7]--> pos z-order.
-//                       |     |
-//        Neg z-order. <-+     +--> Normal flow descendants.
-//
-// We can then assign new, 'stacking' order indices to these elements as follows:
-//
-//                     [-8], [A-D], [0, 2, 5, 7]
-// 'Stacking' indices:  -1     0     1  2  3  4
-//
-// Note that the normal flow descendants can share an index because they don't
-// stack/overlap. Now our problem becomes very simple: a layer can safely become
-// a stacking container if the stacking-order indices of it and its descendants
-// appear in a contiguous block in the list of stacking indices. This problem
-// can be solved very efficiently by calculating the min/max stacking indices in
-// the subtree, and the number stacking container descendants. Once we have this
-// information, we know that the subtree's indices form a contiguous block if:
-//
-//           maxStackIndex - minStackIndex == numSCDescendants
+static inline bool isPositionedContainer(const RenderLayer* layer)
+{
+    // FIXME: This is not in sync with containingBlock.
+    // RenderObject::canContainFixedPositionedObject() should probably be used
+    // instead.
+    RenderLayerModelObject* layerRenderer = layer->renderer();
+    return layer->isRootLayer() || layerRenderer->isPositioned() || layer->hasTransform();
+}
+
+enum StackingOrderDirection { FromBackground, FromForeground };
+
+// We'd like to be able to iterate through a single paint order list, but for
+// efficiency's sake, we hang onto two lists instead (namely, the pos and neg
+// z-order lists produced by CollectLayers). This function allows us to index
+// into these two lists as if they were one. It also allows us to index into
+// this virtual list either from the start or from the end (i.e., in either
+// stacking order direction).
+static const RenderLayer* getStackingOrderElementAt(const Vector<RenderLayer*>* posZOrderList, const Vector<RenderLayer*>* negZOrderList, const StackingOrderDirection direction, const size_t index)
+{
+    size_t negZOrderListSize = negZOrderList ? negZOrderList->size() : 0;
+
+    if (direction == FromBackground) {
+        if (index < negZOrderListSize)
+            return negZOrderList->at(index);
+
+        return posZOrderList->at(index - negZOrderListSize);
+    }
+
+    size_t posZOrderListSize = posZOrderList ? posZOrderList->size() : 0;
+
+    if (index < posZOrderListSize)
+        return posZOrderList->at(posZOrderListSize - index - 1);
+
+    return negZOrderList->at(negZOrderListSize - (index - posZOrderListSize) - 1);
+}
+
+// After promotion, the paint order consists of
+// a) the non-descendants which precede the promoted layer,
+// b) the promoted layer, and
+// c) the non-descendants which succeed the promoted layer.
 //
-// So for node A in the example above we would have:
-//   maxStackIndex = 1
-//   minStackIndex = -1
-//   numSCDecendants = 2
+// If the current layer's descendants form a contiguous block in paint order
+// before promotion, the paint order will consist of
+// a) the non-descendants which precede the current layer and its descendants,
+// b) the current layer and its descendants
+// c) The non-descendants which succeed the current layer and its descendants.
 //
-// and so,
-//       maxStackIndex - minStackIndex == numSCDescendants
-//  ===>                      1 - (-1) == 2
-//  ===>                             2 == 2
+// Sub-lists (a) and (c) should be identical in both paint order lists if
+// and only if the descendants form a contiguous block. In fact, this is the
+// only check we need to perform since the order of the descendants with
+// respect to each other cannot be affected by promotion (i.e., we don't
+// need to worry about sub-list (b)).
 //
-//  Since this is true, A can safely become a stacking container.
-//  Now, for node C we have:
+// Some examples:
+// C = currentLayer
+// - = negative z-order child of currentLayer
+// + = positive z-order child of currentLayer
+// A = positioned ancestor of currentLayer
+// x = any other RenderLayer in the list
 //
-//   maxStackIndex = 4
-//   minStackIndex = 0 <-- because C has stacking index 0.
-//   numSCDecendants = 2
+//        original                    |   zOrderListBeforePromote    |  zOrderListAfterPromote
+//        zOrderListBeforePromote     |  after inserting C as above  |
+//     ---------------------------------------------------------------------------------------
+// (1)     x---+++x                   |  x---C+++x                   |     xCx
+// (2)     x---+A++x                  |  x---+AC++x                  |     xACx
+// (3)     x-x--+++x                  |  x-x--C+++x                  |     xxCx
+// (4)     xxA++x                     |  xxAC++x                     |     xxACx
 //
-// and so,
-//       maxStackIndex - minStackIndex == numSCDescendants
-//  ===>                         4 - 0 == 2
-//  ===>                             4 == 2
+// In example (1), we compare sub-list (a) by marching from the left of both
+// lists (zOrderListBeforePromote after inserting C and
+// zOrderListAfterPromote). The first mismatch is at index 1 when we hit '-'
+// and 'C'. That means we have neg z-order descendants. This is a problem if
+// we have a background. Before promotion, this bg would get painted with
+// the current layer (i.e., after the neg z-order descendants), but after
+// promotion the bg would get painted before them. This is a stacking order
+// violation and we can't promote. However, if we don't have a background,
+// we would continue on to the second pass. When comparing from the right,
+// we mismatch on '+' and 'C'. Since we hit 'C' on zOrderListAfterPromote,
+// we know that the children are contiguous, and we will promote.
 //
-// Since this is false, C cannot be safely promoted to a stacking container. This
-// happened because of the elements with z-index 5 and 0. Now if 5 had been a
-// child of C rather than D, and A had no child with Z index 0, we would have had:
+// In example (2), when marching from the left, we'll hit a mismatch again
+// on the second element we look at. This time, since this element is an 'A'
+// in zOrderListAfterPromote, this indicates that there is an extra layer
+// (the 'A') mixed in with the children. This could cause a change in paint
+// order if we promote, so we decide not to and break out of the loop. Note
+// that if the current layer has a background, this would provide a second
+// reason not to opt in, since again we have negative z-order children who
+// would change paint order with respect to our background if we promoted.
 //
-//   maxStackIndex = 3
-//   minStackIndex = 0 <-- because C has stacking index 0.
-//   numSCDecendants = 3
+// In example (3), the discontiguity of the negative z-order children causes
+// us to fail early in our "FromBackground" pass when we try to compare '-'
+// from zOrderListBeforePromote with 'x' in zOrderListAfterPromote.
 //
-// and so,
-//       maxStackIndex - minStackIndex == numSCDescendants
-//  ===>                         3 - 0 == 3
-//  ===>                             3 == 3
+// Finally in example (4), we would match 'xxAC' from the left, then stop
+// since we hit 'C'. Then we would match 'x' from the right, and mismatch
+// on '+' and 'C'. Since we're at 'C' on the zOrderListAfterPromote, we
+// conclude that all the children are contiguous. Since there are no
+// negative z-order children, a background layer is irrelevant in this case.
+// We will opt in, keeping paint order constant.
+static bool compareLayerListsBeforeAndAfterPromote(const RenderLayer* currentLayer,
+    const Vector<RenderLayer*>* posZOrderListBeforePromote,
+    const Vector<RenderLayer*>* negZOrderListBeforePromote,
+    const Vector<RenderLayer*>* posZOrderListAfterPromote,
+    const Vector<RenderLayer*>* negZOrderListAfterPromote,
+    const size_t sizeBeforePromote,
+    const size_t sizeAfterPromote,
+    const StackingOrderDirection direction)
+{
+    for (size_t index = 0; index < sizeBeforePromote && index < sizeAfterPromote; index++) {
+        const RenderLayer* layerBeforePromote = getStackingOrderElementAt(posZOrderListBeforePromote, negZOrderListBeforePromote, direction, index);
+        const RenderLayer* layerAfterPromote = getStackingOrderElementAt(posZOrderListAfterPromote, negZOrderListAfterPromote, direction, index);
+
+        if (layerBeforePromote != layerAfterPromote) {
+            // If we find a mismatch, the only situation where we haven't
+            // necessarily changed paint order yet is if layerAfterPromote
+            // is currentLayer.
+            if (layerAfterPromote != currentLayer)
+                return false;
+
+            // Also, if the current layer has a background, then any
+            // negative z-order children will get between the background
+            // and the rest of the layer.
+            if (direction == FromBackground && currentLayer->renderer()->hasBackground())
+                return false;
+        }
+
+        // To compare the sub-lists (a) and (c) from the comment above, we only
+        // need to march until we hit the currentLayer in the
+        // zOrderListAfterPromote from each direction.
+        if (layerAfterPromote == currentLayer)
+            break;
+    }
+
+    return true;
+}
+
+// Determine whether the current layer can be promoted to a stacking container,
+// given its closest stacking context ancestor. We do this by computing what
+// positive and negative z-order lists would look like before and after
+// promotion, and ensuring that proper stacking order is preserved between the
+// two sets of lists.
 //
-//  And we would conclude that C could be promoted.
-void RenderLayer::updateDescendantsAreContiguousInStackingOrder()
+// For more details on how the lists will be compared, see the comment and
+// examples for compareLayerListsBeforeAndAfterPromote().
+void RenderLayer::updateCanBeStackingContainer(RenderLayer* ancestorStackingContext)
 {
-    if (!m_descendantsAreContiguousInStackingOrderDirty || !isStackingContext() || !acceleratedCompositingForOverflowScrollEnabled())
+    TRACE_EVENT0("blink", "RenderLayer::updateCanBeStackingContainer");
+    ASSERT(!isStackingContext());
+
+    if (m_updatingCanBePromotedToStackingContainer || !acceleratedCompositingForOverflowScrollEnabled())
         return;
 
-    ASSERT(!m_normalFlowListDirty);
-    ASSERT(!m_zOrderListsDirty);
+    FrameView* frameView = renderer()->view()->frameView();
+    if (!frameView || !frameView->containsScrollableArea(this))
+        return;
 
-    OwnPtr<Vector<RenderLayer*> > posZOrderList;
-    OwnPtr<Vector<RenderLayer*> > negZOrderList;
-    rebuildZOrderLists(StopAtStackingContexts, posZOrderList, negZOrderList);
+    m_updatingCanBePromotedToStackingContainer = true;
 
-    // Create a reverse lookup.
-    HashMap<const RenderLayer*, int> lookup;
+    OwnPtr<Vector<RenderLayer*> > posZOrderListBeforePromote;
+    OwnPtr<Vector<RenderLayer*> > negZOrderListBeforePromote;
+    OwnPtr<Vector<RenderLayer*> > posZOrderListAfterPromote;
+    OwnPtr<Vector<RenderLayer*> > negZOrderListAfterPromote;
+    size_t posZOrderListSizeBeforePromote, negZOrderListSizeBeforePromote, posZOrderListSizeAfterPromote, negZOrderListSizeAfterPromote;
 
-    if (negZOrderList) {
-        int stackingOrderIndex = -1;
-        size_t listSize = negZOrderList->size();
-        for (size_t i = 0; i < listSize; ++i) {
-            RenderLayer* currentLayer = negZOrderList->at(listSize - i - 1);
-            if (!currentLayer->isStackingContext())
-                continue;
-            lookup.set(currentLayer, stackingOrderIndex--);
-        }
-    }
+    collectBeforePromotionZOrderList(ancestorStackingContext, posZOrderListBeforePromote, negZOrderListBeforePromote);
+    collectAfterPromotionZOrderList(ancestorStackingContext, posZOrderListAfterPromote, negZOrderListAfterPromote);
 
-    if (posZOrderList) {
-        size_t listSize = posZOrderList->size();
-        int stackingOrderIndex = 1;
-        for (size_t i = 0; i < listSize; ++i) {
-            RenderLayer* currentLayer = posZOrderList->at(i);
-            if (!currentLayer->isStackingContext())
-                continue;
-            lookup.set(currentLayer, stackingOrderIndex++);
-        }
-    }
+    size_t sizeBeforePromote = 0;
+    if (posZOrderListBeforePromote)
+        sizeBeforePromote += posZOrderListBeforePromote->size();
+    if (negZOrderListBeforePromote)
+        sizeBeforePromote += negZOrderListBeforePromote->size();
 
-    int minIndex = 0;
-    int maxIndex = 0;
-    int count = 0;
-    bool firstIteration = true;
-    updateDescendantsAreContiguousInStackingOrderRecursive(lookup, minIndex, maxIndex, count, firstIteration);
+    size_t sizeAfterPromote = 0;
+    if (posZOrderListAfterPromote)
+        sizeAfterPromote += posZOrderListAfterPromote->size();
+    if (negZOrderListAfterPromote)
+        sizeAfterPromote += negZOrderListAfterPromote->size();
 
-    m_descendantsAreContiguousInStackingOrderDirty = false;
+    bool canPromote = compareLayerListsBeforeAndAfterPromote(this, posZOrderListBeforePromote.get(), negZOrderListBeforePromote.get(),
+        posZOrderListAfterPromote.get(), negZOrderListAfterPromote.get(),
+        sizeBeforePromote, sizeAfterPromote, FromBackground)
+        && compareLayerListsBeforeAndAfterPromote(this, posZOrderListBeforePromote.get(), negZOrderListBeforePromote.get(),
+            posZOrderListAfterPromote.get(), negZOrderListAfterPromote.get(),
+            sizeBeforePromote, sizeAfterPromote, FromForeground);
+
+    bool didUpdate = (canPromote != m_canBePromotedToStackingContainer);
+
+    m_canBePromotedToStackingContainer = canPromote;
+    m_updatingCanBePromotedToStackingContainer = false;
 }
 
-void RenderLayer::updateDescendantsAreContiguousInStackingOrderRecursive(const HashMap<const RenderLayer*, int>& lookup, int& minIndex, int& maxIndex, int& count, bool firstIteration)
+void RenderLayer::updateCanBeStackingContainerRecursively(RenderLayer* ancestorStackingContext)
 {
-    if (isStackingContext() && !firstIteration) {
-        if (lookup.contains(this)) {
-            minIndex = std::min(minIndex, lookup.get(this));
-            maxIndex = std::max(maxIndex, lookup.get(this));
-            count++;
-        }
-        return;
-    }
+    if (this != ancestorStackingContext) {
+        if (isStackingContext())
+            return;
 
-    for (RenderLayer* child = firstChild(); child; child = child->nextSibling()) {
-        int childMinIndex = 0;
-        int childMaxIndex = 0;
-        int childCount = 0;
-        child->updateDescendantsAreContiguousInStackingOrderRecursive(lookup, childMinIndex, childMaxIndex, childCount, false);
-        if (childCount) {
-            count += childCount;
-            minIndex = std::min(minIndex, childMinIndex);
-            maxIndex = std::max(maxIndex, childMaxIndex);
-        }
+        updateCanBeStackingContainer(ancestorStackingContext);
     }
 
-    if (!isStackingContext()) {
-        bool newValue = maxIndex - minIndex == count;
-        bool didUpdate = newValue != m_descendantsAreContiguousInStackingOrder;
-        m_descendantsAreContiguousInStackingOrder = newValue;
-        if (didUpdate)
-            updateNeedsCompositedScrolling();
+    if (m_hasVisibleDescendant) {
+        for (RenderLayer* child = firstChild(); child; child = child->nextSibling()) {
+            // Ignore reflections.
+            if (!m_reflection || reflectionLayer() != child)
+                child->updateCanBeStackingContainerRecursively(ancestorStackingContext);
+        }
     }
 }
 
-static inline bool isPositionedContainer(const RenderLayer* layer)
-{
-    // FIXME: This is not in sync with containingBlock.
-    // RenderObject::canContainFixedPositionedObject() should probably be used
-    // instead.
-    RenderLayerModelObject* layerRenderer = layer->renderer();
-    return layer->isRootLayer() || layerRenderer->isPositioned() || layer->hasTransform();
-}
-
+// Compute what positive and negative z-order lists would look like before and
+// after promotion, so we can later ensure that proper stacking order is
+// preserved between the two sets of lists.
+//
+// A few examples:
+// c = currentLayer
+// - = negative z-order child of currentLayer
+// + = positive z-order child of currentLayer
+// a = positioned ancestor of currentLayer
+// x = any other RenderLayer in the list
+//
+// (a) xxxxx-----++a+++x
+// (b) xxx-----c++++++xx
+//
+//
+// Normally the current layer would be painted in the normal flow list if it
+// doesn't already appear in the positive z-order list. However, in the case
+// that the layer has a positioned ancestor, it will paint directly after the
+// positioned ancestor. In example (a), the current layer would be painted in
+// the middle of its own positive z-order children, so promoting would cause a
+// change in paint order (since a promoted layer will paint all of its positive
+// z-order children strictly after it paints itself).
+//
+// In example (b), it is ok to promote the current layer only if it does not
+// have a background. If it has a background, the background gets painted before
+// the layer's negative z-order children, so again, a promotion would cause a
+// change in paint order (causing the background to get painted after the
+// negative z-order children instead of before).
 void RenderLayer::collectBeforePromotionZOrderList(RenderLayer* ancestorStackingContext, OwnPtr<Vector<RenderLayer*> >& posZOrderListBeforePromote, OwnPtr<Vector<RenderLayer*> >& negZOrderListBeforePromote)
 {
-    // FIXME: TemporaryChange should support bit fields.
+    // We can't use TemporaryChange<> here since m_needsCompositedScrolling and
+    // m_isNormalFlowOnly are both bitfields, so we have to do it the
+    // old-fashioned way.
     bool oldNeedsCompositedScrolling = m_needsCompositedScrolling;
     bool oldIsNormalFlowOnly = m_isNormalFlowOnly;
 
@@ -1062,7 +1141,7 @@ bool RenderLayer::canBeStackingContainer() const
     if (isStackingContext() || !ancestorStackingContainer())
         return true;
 
-    return m_descendantsAreContiguousInStackingOrder;
+    return m_canBePromotedToStackingContainer;
 }
 
 void RenderLayer::setHasVisibleContent()
@@ -1118,56 +1197,62 @@ void RenderLayer::setAncestorChainHasVisibleDescendant()
     }
 }
 
-void RenderLayer::updateDescendantDependentFlags(HashSet<const RenderObject*>* outOfFlowDescendantContainingBlocks)
+void RenderLayer::updateHasOutOfFlowPositionedDescendant(HashSet<const RenderObject*>* containingBlocks)
 {
-    if (m_visibleDescendantStatusDirty || m_hasSelfPaintingLayerDescendantDirty || m_hasOutOfFlowPositionedDescendantDirty) {
-        const bool hadVisibleDescendant = m_hasVisibleDescendant;
-        const bool hadOutOfFlowPositionedDescendant = m_hasOutOfFlowPositionedDescendant;
+    const bool hadOutOfFlowPositionedDescendant = hasOutOfFlowPositionedDescendant();
+    m_hasOutOfFlowPositionedDescendant = false;
+
+    HashSet<const RenderObject*> childContainingBlocks;
+    for (RenderLayer* child = firstChild(); child; child = child->nextSibling()) {
+        childContainingBlocks.clear();
+        child->updateHasOutOfFlowPositionedDescendant(&childContainingBlocks);
+
+        const bool childIsOutOfFlowPositioned = child->renderer()
+            && child->renderer()->isOutOfFlowPositioned()
+            && (child->hasVisibleDescendant() || child->hasVisibleContent());
+
+        if (childIsOutOfFlowPositioned) {
+            childContainingBlocks.add(child->renderer()->containingBlock());
+
+        childContainingBlocks.remove(renderer());
+
+        if (containingBlocks && !childContainingBlocks.isEmpty()) {
+            HashSet<const RenderObject*>::const_iterator it = childContainingBlocks.begin();
+            for (; it != childContainingBlocks.end(); ++it)
+                containingBlocks->add(*it);
+        }
+
+        m_hasOutOfFlowPositionedDescendant |= !childContainingBlocks.isEmpty();
+    }
 
+    if (m_hasOutOfFlowPositionedDescendant != hadOutOfFlowPositionedDescendant)
+        compositor()->setNeedsUpdateCompositingRequirementsState();
+}
+
+void RenderLayer::updateDescendantDependentFlags()
+{
+    if (m_visibleDescendantStatusDirty || m_hasSelfPaintingLayerDescendantDirty) {
         m_hasVisibleDescendant = false;
         m_hasSelfPaintingLayerDescendant = false;
-        m_hasOutOfFlowPositionedDescendant = false;
 
-        HashSet<const RenderObject*> childOutOfFlowDescendantContainingBlocks;
         for (RenderLayer* child = firstChild(); child; child = child->nextSibling()) {
-            childOutOfFlowDescendantContainingBlocks.clear();
-            child->updateDescendantDependentFlags(&childOutOfFlowDescendantContainingBlocks);
-
-            bool childIsOutOfFlowPositioned = child->renderer() && child->renderer()->isOutOfFlowPositioned();
-            if (childIsOutOfFlowPositioned)
-                childOutOfFlowDescendantContainingBlocks.add(child->renderer()->containingBlock());
-
-            if (outOfFlowDescendantContainingBlocks) {
-                HashSet<const RenderObject*>::const_iterator it = childOutOfFlowDescendantContainingBlocks.begin();
-                for (; it != childOutOfFlowDescendantContainingBlocks.end(); ++it)
-                    outOfFlowDescendantContainingBlocks->add(*it);
-            }
+            child->updateDescendantDependentFlags();
 
             bool hasVisibleDescendant = child->m_hasVisibleContent || child->m_hasVisibleDescendant;
             bool hasSelfPaintingLayerDescendant = child->isSelfPaintingLayer() || child->hasSelfPaintingLayerDescendant();
-            bool hasOutOfFlowPositionedDescendant = hasVisibleDescendant && (!childOutOfFlowDescendantContainingBlocks.isEmpty() || child->hasOutOfFlowPositionedDescendant());
 
             m_hasVisibleDescendant |= hasVisibleDescendant;
             m_hasSelfPaintingLayerDescendant |= hasSelfPaintingLayerDescendant;
-            m_hasOutOfFlowPositionedDescendant |= hasOutOfFlowPositionedDescendant;
 
-            if (m_hasVisibleDescendant && m_hasSelfPaintingLayerDescendant && m_hasOutOfFlowPositionedDescendant)
+            if (m_hasVisibleDescendant && m_hasSelfPaintingLayerDescendant)
                 break;
         }
 
-        if (outOfFlowDescendantContainingBlocks && renderer())
-            outOfFlowDescendantContainingBlocks->remove(renderer());
-
         m_visibleDescendantStatusDirty = false;
         m_hasSelfPaintingLayerDescendantDirty = false;
-        m_hasOutOfFlowPositionedDescendantDirty = false;
-
-        if (m_hasVisibleDescendant != hadVisibleDescendant || m_hasOutOfFlowPositionedDescendant != hadOutOfFlowPositionedDescendant)
-            updateNeedsCompositedScrolling();
     }
 
     if (m_visibleContentStatusDirty) {
-        const bool hadVisibleContent = m_hasVisibleContent;
         if (renderer()->style()->visibility() == VISIBLE)
             m_hasVisibleContent = true;
         else {
@@ -1193,10 +1278,8 @@ void RenderLayer::updateDescendantDependentFlags(HashSet<const RenderObject*>* o
                         r = r->nextSibling();
                 }
             }
-        }    
-        m_visibleContentStatusDirty = false; 
-        if (hadVisibleContent != m_hasVisibleContent)
-            updateNeedsCompositedScrolling();
+        }
+        m_visibleContentStatusDirty = false;
     }
 }
 
@@ -1767,16 +1850,6 @@ void RenderLayer::addChild(RenderLayer* child, RenderLayer* beforeChild)
         // case where we're building up generated content layers. This is ok, since the lists will start
         // off dirty in that case anyway.
         child->dirtyStackingContainerZOrderLists();
-
-        // Adding an out of flow positioned descendant can only affect
-        // the opt-in decision for layers beneath and including our
-        // containing block.
-        RenderObject* containingBlock = child->renderer()->containingBlock();
-        for (RenderLayer* layer = child; layer; layer = layer->parent()) {
-            layer->updateNeedsCompositedScrolling();
-            if (layer->renderer() == containingBlock)
-                break;
-        }
     }
 
     child->updateDescendantDependentFlags();
@@ -1787,7 +1860,7 @@ void RenderLayer::addChild(RenderLayer* child, RenderLayer* beforeChild)
         setAncestorChainHasSelfPaintingLayerDescendant();
 
     if (child->renderer() && (child->renderer()->isOutOfFlowPositioned() || child->hasOutOfFlowPositionedDescendant()))
-        setAncestorChainHasOutOfFlowPositionedDescendant(child->renderer()->containingBlock());
+        compositor()->setNeedsUpdateCompositingRequirementsState();
 
     compositor()->layerWasAdded(this, child);
 }
@@ -1815,28 +1888,16 @@ RenderLayer* RenderLayer::removeChild(RenderLayer* oldChild)
         // reattachment process in removeOnlyThisLayer, the layer may already be disconnected
         // from the main layer tree, so we need to null-check the |stackingContainer| value.
         oldChild->dirtyStackingContainerZOrderLists();
-
-        // This could affect whether or not a layer has an out of flow
-        // positioned descendant so we need to schedule some updates.
-        // Removing an out of flow positioned descendant can only affect
-        // the opt-in decision for layers beneath and including the old child's
-        // containing block.
-        RenderObject* containingBlock = oldChild->renderer()->containingBlock();
-        for (RenderLayer* layer = this; layer; layer = layer->parent()) {
-            layer->updateNeedsCompositedScrolling();
-            if (layer->renderer() == containingBlock)
-                break;
-        }
     }
 
-    if ((oldChild->renderer() && oldChild->renderer()->isOutOfFlowPositioned()) || oldChild->hasOutOfFlowPositionedDescendant())
-        dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
-
     oldChild->setPreviousSibling(0);
     oldChild->setNextSibling(0);
     oldChild->setParent(0);
 
     oldChild->updateDescendantDependentFlags();
+    if ((oldChild->renderer() && oldChild->renderer()->isOutOfFlowPositioned()) || oldChild->hasOutOfFlowPositionedDescendant())
+        compositor()->setNeedsUpdateCompositingRequirementsState();
+
     if (oldChild->m_hasVisibleContent || oldChild->m_hasVisibleDescendant)
         dirtyAncestorChainVisibleDescendantStatus();
 
@@ -2051,20 +2112,47 @@ bool RenderLayer::usesCompositedScrolling() const
 
 bool RenderLayer::needsCompositedScrolling() const
 {
+    switch (m_forceNeedsCompositedScrolling) {
+    case DoNotForceCompositedScrolling:
+        return m_needsCompositedScrolling;
+    case ForceCompositedScrollingOn:
+        return true;
+    case ForceCompositedScrollingOff:
+        return false;
+    }
     return m_needsCompositedScrolling;
 }
 
 void RenderLayer::updateNeedsCompositedScrolling()
 {
-    bool needsCompositedScrolling = false;
+    TRACE_EVENT0("blink-rendering", "RenderLayer::updateNeedsCompositedScrolling\n");

[hartmanng, 2013/05/02 14:04:01]

Might not be a bad idea to leave these trace events in?

+#ifndef NDEBUG
+    fprintf(stderr, "vollick: RenderLayer::updateNeedsCompositedScrolling %s\n", debugName().ascii().data());
+#endif
+    if (m_updatingNeedsCompositedScrolling || m_updatingCanBePromotedToStackingContainer)
+        return;
+
+    bool needsCompositedScrolling = isStackingContext();
 
     FrameView* frameView = renderer()->view()->frameView();
-    if (frameView && frameView->containsScrollableArea(this)) {
+    if (!needsCompositedScrolling && acceleratedCompositingForOverflowScrollEnabled() && frameView && frameView->containsScrollableArea(this)) {
+        m_updatingNeedsCompositedScrolling = true;
         updateDescendantDependentFlags();
+        RenderLayer* stackingContext = ancestorStackingContext();
+        updateCanBeStackingContainer(stackingContext);
 
-        bool forceUseCompositedScrolling = acceleratedCompositingForOverflowScrollEnabled()
-            && canBeStackingContainer()
-            && !hasOutOfFlowPositionedDescendant();
+        bool forceUseCompositedScrolling = canBeStackingContainer() && !hasOutOfFlowPositionedDescendant();
+
+#ifndef NDEBUG
+        fprintf(stderr, "vollick: RenderLayer::updateNeedsCompositedScrolling %d %d %s\n", canBeStackingContainer(), hasOutOfFlowPositionedDescendant(), debugName().ascii().data());
+#endif
+
+        TRACE_EVENT_INSTANT2("blink-rendering",
+                             "RenderLayer::updateNeedsCompositedScrolling",
+                             "Can be a stacking container",
+                             canBeStackingContainer(),
+                             "Has out of flow positioned descendant",
+                             hasOutOfFlowPositionedDescendant());
 
 #if ENABLE(ACCELERATED_OVERFLOW_SCROLLING)
         needsCompositedScrolling = forceUseCompositedScrolling || renderer()->style()->useTouchOverflowScrolling();
@@ -2077,13 +2165,35 @@ void RenderLayer::updateNeedsCompositedScrolling()
         // number of composited overflow divs.
         if (acceleratedCompositingForOverflowScrollEnabled())
             HistogramSupport::histogramEnumeration("Renderer.NeedsCompositedScrolling", needsCompositedScrolling, 2);
+
+        m_updatingNeedsCompositedScrolling = false;
     }
 
+    setNeedsCompositedScrolling(needsCompositedScrolling);
+}
+
+void RenderLayer::setNeedsCompositedScrolling(bool needsCompositedScrolling)
+{
     if (m_needsCompositedScrolling == needsCompositedScrolling)
         return;
 
     m_needsCompositedScrolling = needsCompositedScrolling;
 
+    // Note, the z-order lists may need to be rebuilt, but our code guarantees
+    // that we have not affected stacking, so we will not dirty
+    // m_canBePromotedToStackingContainer for either us or our stacking context
+    // or container.
+    didSetNeedsCompositedScrolling();
+}
+
+void RenderLayer::setForceNeedsCompositedScrolling(RenderLayer::ForceNeedsCompositedScrollingMode mode)
+{
+    m_forceNeedsCompositedScrolling = mode;
+    didSetNeedsCompositedScrolling();
+}
+
+void RenderLayer::didSetNeedsCompositedScrolling()
+{
     updateIsNormalFlowOnly();
     updateSelfPaintingLayer();
 
@@ -5607,7 +5717,7 @@ void RenderLayer::dirtyZOrderLists()
         m_negZOrderList->clear();
     m_zOrderListsDirty = true;
 
-    m_descendantsAreContiguousInStackingOrderDirty = true;
+    compositor()->setNeedsUpdateCompositingRequirementsState();
 
     if (!renderer()->documentBeingDestroyed()) {
         compositor()->setCompositingLayersNeedRebuild();
@@ -5712,18 +5822,28 @@ void RenderLayer::collectLayers(bool includeHiddenLayers, CollectLayersBehavior
     bool isStacking = false;
 
     switch (behavior) {
-        case StopAtStackingContexts:
-            isStacking = (this == layerToForceAsStackingContainer) || isStackingContext();
-            break;
+    case StopAtStackingContexts:
+        isStacking = (this == layerToForceAsStackingContainer) || isStackingContext();
+        break;
 
-        case StopAtStackingContainers:
-            isStacking = (this == layerToForceAsStackingContainer) || isStackingContainer();
-            break;
+    case StopAtStackingContainers:
+         isStacking = (this == layerToForceAsStackingContainer) || isStackingContainer();
+        break;
+    }
+
+    // This value could be affected by opt in. What we really need to know is
+    // are you normal flow only, regardless of your opt in status. This is very
+    // similar to asking if a layer is a stacking context rather than a
+    // stacking container.
+    bool isNormalFlowWithoutCompositedScrolling = isNormalFlowOnly();
+    if (this != layerToForceAsStackingContainer && behavior == StopAtStackingContexts) {
+        TemporaryChange<ForceNeedsCompositedScrollingMode> forceOff(m_forceNeedsCompositedScrolling, ForceCompositedScrollingOff);
+        isNormalFlowWithoutCompositedScrolling = shouldBeNormalFlowOnly();
     }
 
     // Overflow layers are just painted by their enclosing layers, so they don't get put in zorder lists.
     bool includeHiddenLayer = includeHiddenLayers || (m_hasVisibleContent || (m_hasVisibleDescendant && isStacking));
-    if (includeHiddenLayer && !isNormalFlowOnly() && !isOutOfFlowRenderFlowThread()) {
+    if (includeHiddenLayer && !isNormalFlowWithoutCompositedScrolling && !isOutOfFlowRenderFlowThread()) {
         // Determine which buffer the child should be in.
         OwnPtr<Vector<RenderLayer*> >& buffer = (zIndex() >= 0) ? posBuffer : negBuffer;
 
@@ -5748,7 +5868,8 @@ void RenderLayer::collectLayers(bool includeHiddenLayers, CollectLayersBehavior
 
 void RenderLayer::updateLayerListsIfNeeded()
 {
-    bool shouldUpdateDescendantsAreContiguousInStackingOrder = acceleratedCompositingForOverflowScrollEnabled() && isStackingContext() && (m_zOrderListsDirty || m_normalFlowListDirty) && m_descendantsAreContiguousInStackingOrderDirty;
+//    bool shouldUpdateCanBeStackingContainer = acceleratedCompositingForOverflowScrollEnabled() && isStackingContext() && (m_zOrderListsDirty || m_normalFlowListDirty) && m_canBePromotedToStackingContainerDirty;
+    bool shouldUpdateCanBeStackingContainer = false;
     updateZOrderLists();
     updateNormalFlowList();
 
@@ -5757,8 +5878,11 @@ void RenderLayer::updateLayerListsIfNeeded()
         reflectionLayer->updateNormalFlowList();
     }
 
-    if (shouldUpdateDescendantsAreContiguousInStackingOrder) {
-        updateDescendantsAreContiguousInStackingOrder();
+    if (shouldUpdateCanBeStackingContainer) {
+        // Call UpdateCanBeStackingContainer for all descendants,
+        // passing self in as ancestor stacking context.
+        updateCanBeStackingContainerRecursively(this);
+
         // The above function can cause us to update m_needsCompositedScrolling
         // and dirty our layer lists. Refresh them if necessary.
         updateZOrderLists();
@@ -5943,9 +6067,7 @@ void RenderLayer::updateVisibilityAfterStyleChange(const RenderStyle* oldStyle)
         return;
 
     if (renderer()->style()->visibility() == VISIBLE)
-        setAncestorChainHasOutOfFlowPositionedDescendant(renderer()->containingBlock());
-    else
-        dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
+        compositor()->setNeedsUpdateCompositingRequirementsState();
 }
 
 void RenderLayer::updateStackingContextsAfterStyleChange(const RenderStyle* oldStyle)
@@ -5967,7 +6089,10 @@ void RenderLayer::updateStackingContextsAfterStyleChange(const RenderStyle* oldS
     else
         clearZOrderLists();
 
-    updateNeedsCompositedScrolling();
+    // FIXME: This should be done via a function so that we can flip a bit on
+    // the renderview allowing us to skip updates in frames where no comp-scroll
+    // state actually changed.
+    compositor()->setNeedsUpdateCompositingRequirementsState();
 }
 
 static bool overflowRequiresScrollbar(EOverflow overflow)
@@ -6016,46 +6141,10 @@ void RenderLayer::updateScrollbarsAfterStyleChange(const RenderStyle* oldStyle)
         updateScrollableAreaSet(hasScrollableHorizontalOverflow() || hasScrollableVerticalOverflow());
 }
 
-void RenderLayer::setAncestorChainHasOutOfFlowPositionedDescendant(RenderObject* containingBlock)
-{
-    for (RenderLayer* layer = this; layer; layer = layer->parent()) {
-        if (!layer->m_hasOutOfFlowPositionedDescendantDirty && layer->hasOutOfFlowPositionedDescendant())
-            break;
-
-        layer->m_hasOutOfFlowPositionedDescendantDirty = false;
-        layer->m_hasOutOfFlowPositionedDescendant = true;
-        layer->updateNeedsCompositedScrolling();
-
-        if (layer->renderer() && layer->renderer() == containingBlock)
-            break;
-    }
-}
-
-void RenderLayer::dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus()
-{
-    if (m_hasOutOfFlowPositionedDescendant) {
-        m_hasOutOfFlowPositionedDescendantDirty = true;
-        // FIXME It would be nice to avoid this when we clean up render layer
-        // updating. We shouldn't have to update the composited scrolling state
-        // nearly as frequently if all the updates happen in a single, well
-        // defined phase.
-        updateNeedsCompositedScrolling();
-    }
-
-    if (parent())
-        parent()->dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
-}
-
 void RenderLayer::updateOutOfFlowPositioned(const RenderStyle* oldStyle)
 {
-    bool wasOutOfFlowPositioned = oldStyle && (oldStyle->position() == AbsolutePosition || oldStyle->position() == FixedPosition);
-    bool isOutOfFlowPositioned = renderer()->isOutOfFlowPositioned();
-    if (parent() && isOutOfFlowPositioned != wasOutOfFlowPositioned) {
-        if (isOutOfFlowPositioned)
-            parent()->setAncestorChainHasOutOfFlowPositionedDescendant(renderer()->containingBlock());
-        else
-            parent()->dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
-    }
+    if (!oldStyle || (renderer()->style()->position() != oldStyle->position()))
+        compositor()->setNeedsUpdateCompositingRequirementsState();
 }
 
 static bool hasOrHadFilters(const RenderStyle* oldStyle, const RenderStyle* newStyle)
@@ -6213,7 +6302,7 @@ void RenderLayer::updateScrollableAreaSet(bool hasOverflow)
         updatedScrollableAreaSet = frameView->removeScrollableArea(this);
 
     if (updatedScrollableAreaSet)
-        updateNeedsCompositedScrolling();
+        compositor()->setNeedsUpdateCompositingRequirementsState();
 }
 
 void RenderLayer::updateScrollCornerStyle()