Rework composited scrolling updates.

Source/WebCore/rendering/RenderLayer.cpp

Index: Source/WebCore/rendering/RenderLayer.cpp
diff --git a/Source/WebCore/rendering/RenderLayer.cpp b/Source/WebCore/rendering/RenderLayer.cpp
index c4787b98c1b85f14ef32d7096cf2f4c121f39b59..8cbfb94447a93139debde788f6c752944355f998 100644
--- a/Source/WebCore/rendering/RenderLayer.cpp
+++ b/Source/WebCore/rendering/RenderLayer.cpp
@@ -146,9 +146,10 @@ RenderLayer::RenderLayer(RenderLayerModelObject* renderer)
     , m_hasSelfPaintingLayerDescendant(false)
     , m_hasSelfPaintingLayerDescendantDirty(false)
     , m_hasOutOfFlowPositionedDescendant(false)
-    , m_hasOutOfFlowPositionedDescendantDirty(true)
+    , m_hasOutOfFlowPositionedDescendantDirty(false)
     , m_needsCompositedScrolling(false)
-    , m_descendantsAreContiguousInStackingOrder(false)
+    , m_descendantsAreContiguousInStackingOrder(true)
+    , m_descendantsAreContiguousInStackingOrderDirty(false)
     , m_isRootLayer(renderer->isRenderView())
     , m_usedTransparency(false)
     , m_paintingInsideReflection(false)
@@ -595,11 +596,10 @@ bool RenderLayer::acceleratedCompositingForOverflowScrollEnabled() const
 //  And we would conclude that C could be promoted.
 void RenderLayer::updateDescendantsAreContiguousInStackingOrder()
 {
-    if (!isStackingContext() || !acceleratedCompositingForOverflowScrollEnabled())
+    if (!m_descendantsAreContiguousInStackingOrderDirty || !isStackingContext() || !acceleratedCompositingForOverflowScrollEnabled()) {
+        m_descendantsAreContiguousInStackingOrderDirty = false;
         return;
-
-    ASSERT(!m_normalFlowListDirty);
-    ASSERT(!m_zOrderListsDirty);
+    }
 
     OwnPtr<Vector<RenderLayer*> > posZOrderList;
     OwnPtr<Vector<RenderLayer*> > negZOrderList;
@@ -635,10 +635,14 @@ void RenderLayer::updateDescendantsAreContiguousInStackingOrder()
     int count = 0;
     bool firstIteration = true;
     updateDescendantsAreContiguousInStackingOrderRecursive(lookup, minIndex, maxIndex, count, firstIteration);
+
+    m_descendantsAreContiguousInStackingOrderDirty = false;
 }
 
 void RenderLayer::updateDescendantsAreContiguousInStackingOrderRecursive(const HashMap<const RenderLayer*, int>& lookup, int& minIndex, int& maxIndex, int& count, bool firstIteration)
 {
+    m_descendantsAreContiguousInStackingOrderDirty = false;
+
     if (isStackingContext() && !firstIteration) {
         if (lookup.contains(this)) {
             minIndex = std::min(minIndex, lookup.get(this));
@@ -660,13 +664,8 @@ void RenderLayer::updateDescendantsAreContiguousInStackingOrderRecursive(const H
         }
     }
 
-    if (!isStackingContext()) {
-        bool newValue = maxIndex - minIndex == count;
-        bool didUpdate = newValue != m_descendantsAreContiguousInStackingOrder;
-        m_descendantsAreContiguousInStackingOrder = newValue;
-        if (didUpdate)
-            updateNeedsCompositedScrolling();
-    }
+    if (!isStackingContext())
+        m_descendantsAreContiguousInStackingOrder = maxIndex - minIndex == count;
 }
 
 void RenderLayer::computeRepaintRects(const RenderLayerModelObject* repaintContainer, const RenderGeometryMap* geometryMap)
@@ -971,11 +970,12 @@ bool RenderLayer::canBeStackingContainer() const
     if (isStackingContext() || !stackingContainer())
         return true;
 
+    ASSERT(!m_descendantsAreContiguousInStackingOrderDirty);
     return m_descendantsAreContiguousInStackingOrder;
 }
 
 void RenderLayer::setHasVisibleContent()
-{ 
+{
     if (m_hasVisibleContent && !m_visibleContentStatusDirty) {
         ASSERT(!parent() || parent()->hasVisibleDescendant());
         return;
@@ -988,7 +988,7 @@ void RenderLayer::setHasVisibleContent()
         // We don't collect invisible layers in z-order lists if we are not in compositing mode.
         // As we became visible, we need to dirty our stacking containers ancestors to be properly
         // collected. FIXME: When compositing, we could skip this dirtying phase.
-        for (RenderLayer* sc = stackingContainer(); sc; sc = sc->stackingContainer()) {
+        for (RenderLayer* sc = parent(); sc; sc = sc->parent()) {
             sc->dirtyZOrderLists();
             if (sc->hasVisibleContent())
                 break;
@@ -1051,7 +1051,7 @@ void RenderLayer::updateDescendantDependentFlags(HashSet<const RenderObject*>* o
 
             bool hasVisibleDescendant = child->m_hasVisibleContent || child->m_hasVisibleDescendant;
             bool hasSelfPaintingLayerDescendant = child->isSelfPaintingLayer() || child->hasSelfPaintingLayerDescendant();
-            bool hasOutOfFlowPositionedDescendant = !childOutOfFlowDescendantContainingBlocks.isEmpty();
+            bool hasOutOfFlowPositionedDescendant = !childOutOfFlowDescendantContainingBlocks.isEmpty() || child->hasOutOfFlowPositionedDescendant();
 
             m_hasVisibleDescendant |= hasVisibleDescendant;
             m_hasSelfPaintingLayerDescendant |= hasSelfPaintingLayerDescendant;
@@ -1066,10 +1066,6 @@ void RenderLayer::updateDescendantDependentFlags(HashSet<const RenderObject*>* o
 
         m_visibleDescendantStatusDirty = false;
         m_hasSelfPaintingLayerDescendantDirty = false;
-
-        if (m_hasOutOfFlowPositionedDescendantDirty)
-            updateNeedsCompositedScrolling();
-
         m_hasOutOfFlowPositionedDescendantDirty = false;
     }
 
@@ -1277,13 +1273,13 @@ FloatPoint RenderLayer::perspectiveOrigin() const
                       floatValueForLength(style->perspectiveOriginY(), borderBox.height()));
 }
 
-RenderLayer* RenderLayer::stackingContainer() const
+RenderLayer* RenderLayer::stackingContainer(bool allowDirty) const
 {
     RenderLayer* layer = parent();
-    while (layer && !layer->isStackingContainer())
+    while (layer && !layer->isStackingContainer(allowDirty))
         layer = layer->parent();
 
-    ASSERT(!layer || layer->isStackingContainer());
+    ASSERT(!layer || layer->isStackingContainer(allowDirty));
     return layer;
 }
 
@@ -1331,9 +1327,9 @@ RenderLayer* RenderLayer::enclosingTransformedAncestor() const
     return curr;
 }
 
-static inline const RenderLayer* compositingContainer(const RenderLayer* layer)
+static inline const RenderLayer* compositingContainer(const RenderLayer* layer, bool allowDirty = false)
 {
-    return layer->isNormalFlowOnly() ? layer->parent() : layer->stackingContainer();
+    return layer->isNormalFlowOnly() ? layer->parent() : layer->stackingContainer(allowDirty);
 }
 
 inline bool RenderLayer::shouldRepaintAfterLayout() const
@@ -1365,7 +1361,7 @@ RenderLayer* RenderLayer::enclosingCompositingLayerForRepaint(bool includeSelf)
     if (includeSelf && isComposited() && !backing()->paintsIntoCompositedAncestor())
         return const_cast<RenderLayer*>(this);
 
-    for (const RenderLayer* curr = compositingContainer(this); curr; curr = compositingContainer(curr)) {
+    for (const RenderLayer* curr = compositingContainer(this, true); curr; curr = compositingContainer(curr, true)) {
         if (curr->isComposited() && !curr->backing()->paintsIntoCompositedAncestor())
             return const_cast<RenderLayer*>(curr);
     }
@@ -1677,6 +1673,7 @@ 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();
+        child->m_descendantsAreContiguousInStackingOrderDirty = true;
     }
 
     child->updateDescendantDependentFlags();
@@ -1710,11 +1707,12 @@ RenderLayer* RenderLayer::removeChild(RenderLayer* oldChild)
 
     if (oldChild->isNormalFlowOnly())
         dirtyNormalFlowList();
-    if (!oldChild->isNormalFlowOnly() || oldChild->firstChild()) { 
+    if (!oldChild->isNormalFlowOnly() || oldChild->firstChild()) {
         // Dirty the z-order list in which we are contained.  When called via the
         // 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();
+        m_descendantsAreContiguousInStackingOrderDirty = true;
     }
 
     if ((oldChild->renderer() && oldChild->renderer()->isOutOfFlowPositioned()) || oldChild->hasOutOfFlowPositionedDescendant())
@@ -1723,7 +1721,7 @@ RenderLayer* RenderLayer::removeChild(RenderLayer* oldChild)
     oldChild->setPreviousSibling(0);
     oldChild->setNextSibling(0);
     oldChild->setParent(0);
-    
+
     oldChild->updateDescendantDependentFlags();
     if (oldChild->m_hasVisibleContent || oldChild->m_hasVisibleDescendant)
         dirtyAncestorChainVisibleDescendantStatus();
@@ -1937,48 +1935,67 @@ bool RenderLayer::usesCompositedScrolling() const
     return isComposited() && backing()->scrollingLayer();
 }
 
-bool RenderLayer::needsCompositedScrolling() const
+bool RenderLayer::needsCompositedScrolling(bool allowDirty) const
 {
+    ASSERT(allowDirty || !m_descendantsAreContiguousInStackingOrderDirty);
+    ASSERT(allowDirty || !m_hasOutOfFlowPositionedDescendantDirty);
     return m_needsCompositedScrolling;
 }
 
 void RenderLayer::updateNeedsCompositedScrolling()
 {
-    bool oldNeedsCompositedScrolling = m_needsCompositedScrolling;
+    bool needsCompositedScrolling = false;
 
     FrameView* frameView = renderer()->view()->frameView();
-    if (!frameView || !frameView->containsScrollableArea(this))
-        m_needsCompositedScrolling = false;
-    else {
+    if (frameView && frameView->containsScrollableArea(this)) {
         bool forceUseCompositedScrolling = acceleratedCompositingForOverflowScrollEnabled()
             && canBeStackingContainer()
             && !hasOutOfFlowPositionedDescendant();
 
 #if ENABLE(ACCELERATED_OVERFLOW_SCROLLING)
-        m_needsCompositedScrolling = forceUseCompositedScrolling || renderer()->style()->useTouchOverflowScrolling();
+        needsCompositedScrolling = forceUseCompositedScrolling || renderer()->style()->useTouchOverflowScrolling();
 #else
-        m_needsCompositedScrolling = forceUseCompositedScrolling;
+        needsCompositedScrolling = forceUseCompositedScrolling;
 #endif
         // We gather a boolean value for use with Google UMA histograms to
         // quantify the actual effects of a set of patches attempting to
         // relax composited scrolling requirements, thereby increasing the
         // number of composited overflow divs.
         if (acceleratedCompositingForOverflowScrollEnabled())
-            HistogramSupport::histogramEnumeration("Renderer.NeedsCompositedScrolling", m_needsCompositedScrolling, 2);
+            HistogramSupport::histogramEnumeration("Renderer.NeedsCompositedScrolling", needsCompositedScrolling, 2);
     }
 
-    if (oldNeedsCompositedScrolling != m_needsCompositedScrolling) {
-        updateSelfPaintingLayer();
-        if (isStackingContainer())
-            dirtyZOrderLists();
-        else
-            clearZOrderLists();
+    if (m_needsCompositedScrolling == needsCompositedScrolling)
+        return;
 
-        dirtyStackingContainerZOrderLists();
+    m_needsCompositedScrolling = needsCompositedScrolling;
 
-        compositor()->setShouldReevaluateCompositingAfterLayout();
-        compositor()->setCompositingLayersNeedRebuild();
+    // Update values that depend on m_needsCompositedScrolling.
+    updateIsNormalFlowOnly();
+    updateSelfPaintingLayer();
+
+    // Changes to needsCompositedScrolling will affect which layers appear
+    // in which z-order lists (some layers that had previously been in our
+    // stacking context's lists may now be in ours), but it will never
+    // affect contiguity (this is the point of these opt in checks). So
+    // we will dirty the z-order lists but leave the contiguity dirty bit
+    // unaffected. RenderLayerCompositor guarantees these values are clean
+    // before calling updateNeedsCompositedScrolling, so we just need to
+    // reset the dirty bits to clean.
+    if (isStackingContainer())
+        dirtyZOrderLists();
+    else
+        clearZOrderLists();
+    dirtyStackingContainerZOrderLists();
+
+    for (RenderLayer* layer = this; layer; layer = layer->parent()) {
+        layer->m_descendantsAreContiguousInStackingOrderDirty = false;
+        if (layer->isStackingContext())
+            break;
     }
+
+    compositor()->setShouldReevaluateCompositingAfterLayout();
+    compositor()->setCompositingLayersNeedRebuild();
 }
 
 static inline int adjustedScrollDelta(int beginningDelta) {
@@ -2273,7 +2290,7 @@ void RenderLayer::updateCompositingLayersAfterScroll()
         // repositioning, so update compositing layers from there.
         if (RenderLayer* compositingAncestor = stackingContainer()->enclosingCompositingLayer()) {
             if (compositor()->compositingConsultsOverlap()) {
-                if (usesCompositedScrolling() && !hasOutOfFlowPositionedDescendant())
+                if (usesCompositedScrolling())
                     compositor()->updateCompositingLayers(CompositingUpdateOnCompositedScroll, compositingAncestor);
                 else
                     compositor()->updateCompositingLayers(CompositingUpdateOnScroll, compositingAncestor);
@@ -3108,10 +3125,6 @@ void RenderLayer::updateScrollInfoAfterLayout()
 
     if (originalScrollOffset != scrollOffset())
         scrollToOffsetWithoutAnimation(IntPoint(scrollOffset()));
-
-    // Composited scrolling may need to be enabled or disabled if the amount of overflow changed.
-    if (renderer()->view() && compositor()->updateLayerCompositingState(this))
-        compositor()->setCompositingLayersNeedRebuild();
 }
 
 bool RenderLayer::overflowControlsIntersectRect(const IntRect& localRect) const
@@ -5496,7 +5509,6 @@ static inline bool compareZIndex(RenderLayer* first, RenderLayer* second)
 void RenderLayer::dirtyZOrderLists()
 {
     ASSERT(m_layerListMutationAllowed);
-    ASSERT(isStackingContainer());
 
     if (m_posZOrderList)
         m_posZOrderList->clear();
@@ -5504,6 +5516,8 @@ void RenderLayer::dirtyZOrderLists()
         m_negZOrderList->clear();
     m_zOrderListsDirty = true;
 
+    m_descendantsAreContiguousInStackingOrderDirty = true;
+
     if (!renderer()->documentBeingDestroyed()) {
         compositor()->setCompositingLayersNeedRebuild();
         if (acceleratedCompositingForOverflowScrollEnabled())
@@ -5513,9 +5527,23 @@ void RenderLayer::dirtyZOrderLists()
 
 void RenderLayer::dirtyStackingContainerZOrderLists()
 {
-    RenderLayer* sc = stackingContainer();
-    if (sc)
-        sc->dirtyZOrderLists();
+    // When this function is called, we need to dirty both our enclosing
+    // stacking container and our stacking context's z-order lists since a
+    // z-order change in the container may affect our decision to opt into
+    // composited scrolling and could therefore affect either layer's z-order
+    // lists. Unfortunately, we are not able to ask if a layer is a stacking
+    // container once z-order lists start getting dirtied (since this will
+    // dirty descendants-are-contiguous-in-stacking-order and could affect our
+    // composited scrolling status and consequently our stacking-containerness)
+    // That said, we do know that if we march up our ancestors, once we reach
+    // a stacking context, we must have hit a stacking container (since
+    // stacking contexts are always stacking containers), so it's safe to stop
+    // there.
+    for (RenderLayer* layer = parent(); layer; layer = layer->parent()) {
+        layer->dirtyZOrderLists();
+        if (layer->isStackingContext())
+            break;
+    }
 }
 
 void RenderLayer::dirtyNormalFlowList()
@@ -5526,6 +5554,7 @@ void RenderLayer::dirtyNormalFlowList()
         m_normalFlowList->clear();
     m_normalFlowListDirty = true;
 
+    m_descendantsAreContiguousInStackingOrderDirty = true;
     if (!renderer()->documentBeingDestroyed()) {
         compositor()->setCompositingLayersNeedRebuild();
         if (acceleratedCompositingForOverflowScrollEnabled())
@@ -5628,7 +5657,7 @@ void RenderLayer::collectLayers(bool includeHiddenLayers, CollectLayersBehavior
 
 void RenderLayer::updateLayerListsIfNeeded()
 {
-    bool shouldUpdateDescendantsAreContiguousInStackingOrder = isStackingContext() && (m_zOrderListsDirty || m_normalFlowListDirty);
+    bool shouldUpdateDescendantsAreContiguousInStackingOrder = acceleratedCompositingForOverflowScrollEnabled() && isStackingContext() && (m_zOrderListsDirty || m_normalFlowListDirty) && m_descendantsAreContiguousInStackingOrderDirty;
     updateZOrderLists();
     updateNormalFlowList();
 
@@ -5724,6 +5753,19 @@ bool RenderLayer::shouldBeNormalFlowOnly() const
             ;
 }
 
+void RenderLayer::updateIsNormalFlowOnly()
+{
+    bool isNormalFlowOnly = shouldBeNormalFlowOnly();
+    if (isNormalFlowOnly == m_isNormalFlowOnly)
+        return;
+
+    m_isNormalFlowOnly = isNormalFlowOnly;
+    RenderLayer* p = parent();
+    if (p)
+        p->dirtyNormalFlowList();
+    dirtyStackingContainerZOrderLists();
+}
+
 bool RenderLayer::shouldBeSelfPaintingLayer() const
 {
     return !isNormalFlowOnly()
@@ -5808,27 +5850,21 @@ bool RenderLayer::isVisuallyNonEmpty() const
 
 void RenderLayer::updateStackingContextsAfterStyleChange(const RenderStyle* oldStyle)
 {
-    if (!oldStyle)
-        return;
+    bool wasStackingContext = oldStyle ? isStackingContext(oldStyle) : false;
+    EVisibility oldVisibility = oldStyle ? oldStyle->visibility() : VISIBLE;
+    int oldZIndex = oldStyle ? oldStyle->zIndex() : 0;
 
-    bool wasStackingContext = isStackingContext(oldStyle);
+    // FIXME: RenderLayer already handles visibility changes through our visiblity dirty bits. This logic could
+    // likely be folded along with the rest.
     bool isStackingContext = this->isStackingContext();
-    if (isStackingContext != wasStackingContext) {
-        dirtyStackingContainerZOrderLists();
-        if (isStackingContext)
-            dirtyZOrderLists();
-        else
-            clearZOrderLists();
+    if (isStackingContext == wasStackingContext && oldVisibility == renderer()->style()->visibility() && oldZIndex == renderer()->style()->zIndex())
         return;
-    }
 
-    // FIXME: RenderLayer already handles visibility changes through our visiblity dirty bits. This logic could
-    // likely be folded along with the rest.
-    if (oldStyle->zIndex() != renderer()->style()->zIndex() || oldStyle->visibility() != renderer()->style()->visibility()) {
-        dirtyStackingContainerZOrderLists();
-        if (isStackingContext)
-            dirtyZOrderLists();
-    }
+    dirtyStackingContainerZOrderLists();
+    if (isStackingContainer())
+        dirtyZOrderLists();
+    else
+        clearZOrderLists();
 }
 
 static bool overflowRequiresScrollbar(EOverflow overflow)
@@ -5885,7 +5921,6 @@ void RenderLayer::setAncestorChainHasOutOfFlowPositionedDescendant(RenderObject*
 
         layer->m_hasOutOfFlowPositionedDescendantDirty = false;
         layer->m_hasOutOfFlowPositionedDescendant = true;
-        layer->updateNeedsCompositedScrolling();
 
         if (layer->renderer() && layer->renderer() == containingBlock)
             break;
@@ -5894,7 +5929,9 @@ void RenderLayer::setAncestorChainHasOutOfFlowPositionedDescendant(RenderObject*
 
 void RenderLayer::dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus()
 {
-    m_hasOutOfFlowPositionedDescendantDirty = true;
+    if (m_hasOutOfFlowPositionedDescendant)
+        m_hasOutOfFlowPositionedDescendantDirty = true;
+
     if (parent())
         parent()->dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
 }
@@ -5902,10 +5939,12 @@ void RenderLayer::dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus()
 void RenderLayer::updateOutOfFlowPositioned(const RenderStyle* oldStyle)
 {
     bool wasOutOfFlowPositioned = oldStyle && (oldStyle->position() == AbsolutePosition || oldStyle->position() == FixedPosition);
-    if (parent() && ((renderer() && renderer()->isOutOfFlowPositioned()) != wasOutOfFlowPositioned)) {
-        parent()->dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
-        if (!renderer()->documentBeingDestroyed() && acceleratedCompositingForOverflowScrollEnabled())
-            compositor()->setShouldReevaluateCompositingAfterLayout();
+    bool isOutOfFlowPositioned = renderer()->isOutOfFlowPositioned();
+    if (parent() && isOutOfFlowPositioned != wasOutOfFlowPositioned) {
+        if (isOutOfFlowPositioned)
+            parent()->setAncestorChainHasOutOfFlowPositionedDescendant(renderer()->containingBlock());
+        else
+            parent()->dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
     }
 }
 
@@ -5989,14 +6028,8 @@ void RenderLayer::updateFilters(const RenderStyle* oldStyle, const RenderStyle*
 
 void RenderLayer::styleChanged(StyleDifference, const RenderStyle* oldStyle)
 {
-    bool isNormalFlowOnly = shouldBeNormalFlowOnly();
-    if (isNormalFlowOnly != m_isNormalFlowOnly) {
-        m_isNormalFlowOnly = isNormalFlowOnly;
-        RenderLayer* p = parent();
-        if (p)
-            p->dirtyNormalFlowList();
-        dirtyStackingContainerZOrderLists();
-    }
+    // Skip this for now. We'll do the update after layout.
+    // updateIsNormalFlowOnly();
 
     if (renderer()->style()->overflowX() == OMARQUEE && renderer()->style()->marqueeBehavior() != MNONE && renderer()->isBox()) {
         if (!m_marquee)
@@ -6010,9 +6043,12 @@ void RenderLayer::styleChanged(StyleDifference, const RenderStyle* oldStyle)
 
     updateScrollbarsAfterStyleChange(oldStyle);
     updateStackingContextsAfterStyleChange(oldStyle);
+
     // Overlay scrollbars can make this layer self-painting so we need
     // to recompute the bit once scrollbars have been updated.
-    updateSelfPaintingLayer();
+
+    // Skip this for now. We'll do the update after layout.
+    // updateSelfPaintingLayer();
     updateOutOfFlowPositioned(oldStyle);
 
     if (!hasReflection() && m_reflection)
@@ -6047,16 +6083,19 @@ void RenderLayer::styleChanged(StyleDifference, const RenderStyle* oldStyle)
     updateFilters(oldStyle, renderer()->style());
 #endif
 
-    updateNeedsCompositedScrolling();
-
-    const RenderStyle* newStyle = renderer()->style();
-    if (compositor()->updateLayerCompositingState(this)
-        || needsCompositingLayersRebuiltForClip(oldStyle, newStyle)
-        || needsCompositingLayersRebuiltForOverflow(oldStyle, newStyle)
-        || needsCompositingLayersRebuiltForFilters(oldStyle, newStyle, didPaintWithFilters))
-        compositor()->setCompositingLayersNeedRebuild();
-    else if (isComposited())
-        backing()->updateGraphicsLayerGeometry();
+// FIXME(vollick) this requires updates that won't happen until after layout.
+// Will this just fall out of the updateCompositingLayers call that will be
+// coming anyway?
+//
+//    const RenderStyle* newStyle = renderer()->style();
+//    if (compositor()->updateLayerCompositingState(this)
+//        || needsCompositingLayersRebuiltForClip(oldStyle, newStyle)
+//        || needsCompositingLayersRebuiltForOverflow(oldStyle, newStyle)
+//        || needsCompositingLayersRebuiltForFilters(oldStyle, newStyle, didPaintWithFilters))
+//        compositor()->setCompositingLayersNeedRebuild();
+//    else if (isComposited())
+//        backing()->updateGraphicsLayerGeometry();
+//
 }
 
 void RenderLayer::updateScrollableAreaSet(bool hasOverflow)
@@ -6073,8 +6112,10 @@ void RenderLayer::updateScrollableAreaSet(bool hasOverflow)
     if (HTMLFrameOwnerElement* owner = frame->ownerElement())
         isVisibleToHitTest &= owner->renderer() && owner->renderer()->visibleToHitTesting();
 
-    if (hasOverflow && isVisibleToHitTest ? frameView->addScrollableArea(this) : frameView->removeScrollableArea(this))
-        updateNeedsCompositedScrolling();
+    if (hasOverflow && isVisibleToHitTest)
+        frameView->addScrollableArea(this);
+    else
+        frameView->removeScrollableArea(this);
 }
 
 void RenderLayer::updateScrollCornerStyle()