Only update composited-scrolling state once after layout.

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 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 126 matching lines @@
 }
 
 RenderLayer::RenderLayer(RenderLayerModelObject* renderer)
     : m_inResizeMode(false)
     , m_scrollDimensionsDirty(true)
     , m_normalFlowListDirty(true)
     , m_hasSelfPaintingLayerDescendant(false)
     , m_hasSelfPaintingLayerDescendantDirty(false)
     , m_hasOutOfFlowPositionedDescendant(false)
     , m_hasOutOfFlowPositionedDescendantDirty(true)
+    , m_hasUnclippedDescendant(false)
     , m_forceNeedsCompositedScrolling(DoNotForceCompositedScrolling)
     , m_needsCompositedScrolling(false)
     , m_descendantsAreContiguousInStackingOrder(false)
     , m_descendantsAreContiguousInStackingOrderDirty(true)
     , m_isRootLayer(renderer->isRenderView())
     , m_usedTransparency(false)
     , m_paintingInsideReflection(false)
     , m_inOverflowRelayout(false)
     , m_repaintStatus(NeedsNormalRepaint)
     , m_visibleContentStatusDirty(true)
@@ skipping 62 matching lines @@
         if (FrameView* frameView = frame->view())
             frameView->removeScrollableArea(this);
     }
 
     if (!m_renderer->documentBeingDestroyed()) {
         Node* node = m_renderer->node();
         if (node && node->isElementNode())
             toElement(node)->setSavedLayerScrollOffset(m_scrollOffset);
     }
 
+    if (!m_renderer->documentBeingDestroyed())
+        compositor()->removeOutOfFlowPositionedLayer(this);
+
     destroyScrollbar(HorizontalScrollbar);
     destroyScrollbar(VerticalScrollbar);
 
     if (renderer()->frame() && renderer()->frame()->page()) {
         if (ScrollingCoordinator* scrollingCoordinator = renderer()->frame()->page()->scrollingCoordinator())
             scrollingCoordinator->willDestroyScrollableArea(this);
     }
 
     if (m_reflection)
         removeReflection();
@@ skipping 230 matching lines @@
         layer->m_hasSelfPaintingLayerDescendantDirty = true;
         // If we have reached a self-painting layer, we know our parent should have a self-painting descendant
         // in this case, there is no need to dirty our ancestors further.
         if (layer->isSelfPaintingLayer()) {
             ASSERT(!parent() || parent()->m_hasSelfPaintingLayerDescendantDirty || parent()->hasSelfPaintingLayerDescendant());
             break;
         }
     }
 }
 
+void RenderLayer::setAncestorChainHasOutOfFlowPositionedDescendant()
+{
+    for (RenderLayer* layer = this; layer; layer = layer->parent()) {
+        if (!layer->m_hasOutOfFlowPositionedDescendantDirty && layer->hasOutOfFlowPositionedDescendant())
+            break;
+
+        layer->m_hasOutOfFlowPositionedDescendantDirty = false;
+        layer->m_hasOutOfFlowPositionedDescendant = true;
+    }
+}
+
+void RenderLayer::dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus()
+{
+    for (RenderLayer* layer = this; layer; layer = layer->parent()) {
+        layer->m_hasOutOfFlowPositionedDescendantDirty = true;
+
+        // We may or may not have an unclipped descendant. If we do, we'll reset
+        // this to true the next time composited scrolling state is updated.
+        layer->m_hasUnclippedDescendant = false;
+
+        // If we have reached an out of flow positioned layer, we know our parent should have an out-of-flow positioned descendant.
+        // In this case, there is no need to dirty our ancestors further.
+        if (layer->renderer()->isOutOfFlowPositioned()) {
+            ASSERT(!parent() || parent()->m_hasOutOfFlowPositionedDescendantDirty || parent()->hasOutOfFlowPositionedDescendant());
+            break;
+        }
+    }
+}
+
 bool RenderLayer::acceleratedCompositingForOverflowScrollEnabled() const
 {
     return renderer()->frame()
         && renderer()->frame()->page()
         && 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
@@ skipping 69 matching lines @@
 //       maxStackIndex - minStackIndex == numSCDescendants
 //  ===>                         3 - 0 == 3
 //  ===>                             3 == 3
 //
 //  And we would conclude that C could be promoted.
 void RenderLayer::updateDescendantsAreContiguousInStackingOrder()
 {
     if (!m_descendantsAreContiguousInStackingOrderDirty || !isStackingContext() || !acceleratedCompositingForOverflowScrollEnabled())
         return;
 
-    ASSERT(!m_normalFlowListDirty);
-    ASSERT(!m_zOrderListsDirty);
-
     OwnPtr<Vector<RenderLayer*> > posZOrderList;
     OwnPtr<Vector<RenderLayer*> > negZOrderList;
     rebuildZOrderLists(StopAtStackingContexts, posZOrderList, negZOrderList);
 
     // Create a reverse lookup.
     HashMap<const RenderLayer*, int> lookup;
 
     if (negZOrderList) {
         int stackingOrderIndex = -1;
         size_t listSize = negZOrderList->size();
@@ skipping 42 matching lines @@
         int childCount = 0;
         child->updateDescendantsAreContiguousInStackingOrderRecursive(lookup, childMinIndex, childMaxIndex, childCount, false);
         if (childCount) {
             count += childCount;
             minIndex = std::min(minIndex, childMinIndex);
             maxIndex = std::max(maxIndex, childMaxIndex);
         }
     }
 
     if (!isStackingContext()) {
-        bool newValue = maxIndex - minIndex == count;
-        bool didUpdate = newValue != m_descendantsAreContiguousInStackingOrder;
-        m_descendantsAreContiguousInStackingOrder = newValue;
-        if (didUpdate)
-            updateNeedsCompositedScrolling();
+        m_descendantsAreContiguousInStackingOrder = (maxIndex - minIndex) == count;
+        m_descendantsAreContiguousInStackingOrderDirty = false;
     }
 }
 
 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();
@@ skipping 408 matching lines @@
         if (curr == ancestorStackingContainer)
             return;
     }
 }
 
 bool RenderLayer::canBeStackingContainer() const
 {
     if (isStackingContext() || !ancestorStackingContainer())
         return true;
 
+    ASSERT(!m_descendantsAreContiguousInStackingOrderDirty);
     return m_descendantsAreContiguousInStackingOrder;
 }
 
 void RenderLayer::setHasVisibleContent()
 { 
     if (m_hasVisibleContent && !m_visibleContentStatusDirty) {
         ASSERT(!parent() || parent()->hasVisibleDescendant());
         return;
     }
 
@@ skipping 36 matching lines @@
 {
     for (RenderLayer* layer = this; layer; layer = layer->parent()) {
         if (!layer->m_visibleDescendantStatusDirty && layer->hasVisibleDescendant())
             break;
 
         layer->m_hasVisibleDescendant = true;
         layer->m_visibleDescendantStatusDirty = false;
     }
 }
 
-void RenderLayer::updateDescendantDependentFlags(HashSet<const RenderObject*>* outOfFlowDescendantContainingBlocks)
+// This method is called on all out of flow positioned layers. It simply walks
+// up to its containing block marking every layer it passes as having an
+// 'unclipped' descendant. The map is used to prevent wasted walking. If we
+// have already passed layer A heading towards containing block B, there's no
+// need to do it again; we can early out.
+void RenderLayer::updateHasUnclippedDescendant(RenderLayer::AncestorContainingBlockMap& map)

[Julien - ping for review, 2013/05/06 22:38:26]

Wouldn't checking ancestor->m_hasUnclippedDescendant in the loop would alleviate
the need for this map?

Also based on the average render tree depth (~30 I heard), it seems like an
overkill to add this map. Did this function show up on your profile?

It's also misnamed as it's not really a containing block map but an ancestor
map.

[Ian Vollick, 2013/05/08 16:00:07]

I did some measurements and this is indeed premature optimization. On gmail load
without the map we spend 10.88ms in the function (1810 invocations). With the
map we spend 10.73ms in the function (1686 invocations). I've removed the map
and updated the CL description. Sadly, this makes the worst case O(n^2), but I
really don't expect us to get hit this hard in practice.

+{
+    ASSERT(renderer()->isOutOfFlowPositioned());
+    if (!m_hasVisibleContent && !m_hasVisibleDescendant)
+        return;
+
+    const RenderObject* containingBlock = renderer()->containingBlock();
+    for (RenderLayer* ancestor = parent(); ancestor && ancestor->renderer() != containingBlock; ancestor = ancestor->parent()) {
+        AncestorContainingBlockMap::iterator it = map.find(ancestor);
+        if (it == map.end()) {
+            OwnPtr<HashSet<const RenderObject*> > containingBlocks = adoptPtr(new HashSet<const RenderObject*>);
+            containingBlocks->add(containingBlock);
+            map.add(ancestor, containingBlocks.release());
+        } else if (!it->value->contains(containingBlock)) {
+            it->value->add(containingBlock);
+        } else {
+            break;
+        }
+
+        ancestor->m_hasUnclippedDescendant = true;
+    }
+}
+
+void RenderLayer::updateDescendantDependentFlags()
 {
     if (m_visibleDescendantStatusDirty || m_hasSelfPaintingLayerDescendantDirty || m_hasOutOfFlowPositionedDescendantDirty) {
-        const bool hadVisibleDescendant = m_hasVisibleDescendant;
-        const bool hadOutOfFlowPositionedDescendant = m_hasOutOfFlowPositionedDescendant;
-
         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());
+            bool hasOutOfFlowPositionedDescendant = child->renderer()->isOutOfFlowPositioned() || child->hasOutOfFlowPositionedDescendant();
 
             m_hasVisibleDescendant |= hasVisibleDescendant;
             m_hasSelfPaintingLayerDescendant |= hasSelfPaintingLayerDescendant;
             m_hasOutOfFlowPositionedDescendant |= hasOutOfFlowPositionedDescendant;
 
-            if (m_hasVisibleDescendant && m_hasSelfPaintingLayerDescendant && m_hasOutOfFlowPositionedDescendant)
+            if (m_hasVisibleDescendant && m_hasSelfPaintingLayerDescendant && hasOutOfFlowPositionedDescendant)
                 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 {
             // layer may be hidden but still have some visible content, check for this
             m_hasVisibleContent = false;
             RenderObject* r = renderer()->firstChild();
             while (r) {
                 if (r->style()->visibility() == VISIBLE && !r->hasLayer()) {
                     m_hasVisibleContent = true;
                     break;
                 }
                 if (r->firstChild() && !r->hasLayer())
                     r = r->firstChild();
                 else if (r->nextSibling())
                     r = r->nextSibling();
                 else {
                     do {
                         r = r->parent();
                         if (r == renderer())
                             r = 0;
                     } while (r && !r->nextSibling());
                     if (r)
                         r = r->nextSibling();
                 }
             }
-        }    
-        m_visibleContentStatusDirty = false; 
-        if (hadVisibleContent != m_hasVisibleContent)
-            updateNeedsCompositedScrolling();
+        }
+        m_visibleContentStatusDirty = false;
     }
 }
 
 void RenderLayer::dirty3DTransformedDescendantStatus()
 {
     RenderLayer* curr = ancestorStackingContainer();
     if (curr)
         curr->m_3DTransformedDescendantStatusDirty = true;
         
     // This propagates up through preserve-3d hierarchies to the enclosing flattening layer.
@@ skipping 550 matching lines @@
     child->setParent(this);
 
     if (child->isNormalFlowOnly())
         dirtyNormalFlowList();
 
     if (!child->isNormalFlowOnly() || child->firstChild()) {
         // Dirty the z-order list in which we are contained. The ancestorStackingContainer() can be null in the
         // 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();
     if (child->m_hasVisibleContent || child->m_hasVisibleDescendant)
         setAncestorChainHasVisibleDescendant();
 
     if (child->isSelfPaintingLayer() || child->hasSelfPaintingLayerDescendant())
         setAncestorChainHasSelfPaintingLayerDescendant();
 
-    if (child->renderer() && (child->renderer()->isOutOfFlowPositioned() || child->hasOutOfFlowPositionedDescendant()))
-        setAncestorChainHasOutOfFlowPositionedDescendant(child->renderer()->containingBlock());
+    if (child->renderer() && (child->renderer()->isOutOfFlowPositioned() || child->hasOutOfFlowPositionedDescendant())) {

[Julien - ping for review, 2013/05/06 22:38:26]

child->renderer()->isOutOfFlowPositioned() ||
child->hasOutOfFlowPositionedDescendant() is repeated several time. It's
probably time to add a helper function.

[Ian Vollick, 2013/05/08 16:00:07]

Done.

+        // Now that the out of flow positioned descendant is in the tree, we
+        // need to tell the compositor to reevaluate the compositing
+        // requirements since we may be able to mark more layers as having
+        // an 'unclipped' descendant.
+        compositor()->setNeedsUpdateCompositingRequirementsState();
+        setAncestorChainHasOutOfFlowPositionedDescendant();
+    }
 
     compositor()->layerWasAdded(this, child);
 }
 
 RenderLayer* RenderLayer::removeChild(RenderLayer* oldChild)
 {
     if (!renderer()->documentBeingDestroyed())
         compositor()->layerWillBeRemoved(this, oldChild);
 
     // remove the child
     if (oldChild->previousSibling())
         oldChild->previousSibling()->setNextSibling(oldChild->nextSibling());
     if (oldChild->nextSibling())
         oldChild->nextSibling()->setPreviousSibling(oldChild->previousSibling());
 
     if (m_first == oldChild)
         m_first = oldChild->nextSibling();
     if (m_last == oldChild)
         m_last = oldChild->previousSibling();
 
     if (oldChild->isNormalFlowOnly())
         dirtyNormalFlowList();
     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();
-
-        // 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()) {
+        // It may now be the case that a layer no longer has an unclipped
+        // descendant. Let the compositor know that it needs to reevaluate
+        // its compositing requirements to check this.
+        compositor()->setNeedsUpdateCompositingRequirementsState();
+        dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
+    }
+
     if (oldChild->m_hasVisibleContent || oldChild->m_hasVisibleDescendant)
         dirtyAncestorChainVisibleDescendantStatus();
 
     if (oldChild->isSelfPaintingLayer() || oldChild->hasSelfPaintingLayerDescendant())
         dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
 
     return oldChild;
 }
 
 void RenderLayer::removeOnlyThisLayer()
@@ skipping 208 matching lines @@
         return true;
     case ForceCompositedScrollingOff:
         return false;
     }
 
     return m_needsCompositedScrolling;
 }
 
 void RenderLayer::updateNeedsCompositedScrolling()
 {
+    if (RenderLayer* ancestor = ancestorStackingContext())
+        ancestor->updateDescendantsAreContiguousInStackingOrder();
+
     bool needsCompositedScrolling = false;
 
     FrameView* frameView = renderer()->view()->frameView();
     if (frameView && frameView->containsScrollableArea(this)) {
         updateDescendantDependentFlags();
 
         bool forceUseCompositedScrolling = acceleratedCompositingForOverflowScrollEnabled()
             && canBeStackingContainer()
-            && !hasOutOfFlowPositionedDescendant();
+            && !hasUnclippedDescendant();
 
 #if ENABLE(ACCELERATED_OVERFLOW_SCROLLING)
         needsCompositedScrolling = forceUseCompositedScrolling || renderer()->style()->useTouchOverflowScrolling();
 #else
         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", needsCompositedScrolling, 2);
     }
 
+    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.
     didUpdateNeedsCompositedScrolling();
@@ skipping 3507 matching lines @@
 
     if (m_posZOrderList)
         m_posZOrderList->clear();
     if (m_negZOrderList)
         m_negZOrderList->clear();
     m_zOrderListsDirty = true;
 
     m_descendantsAreContiguousInStackingOrderDirty = true;
 
     if (!renderer()->documentBeingDestroyed()) {
+        compositor()->setNeedsUpdateCompositingRequirementsState();
         compositor()->setCompositingLayersNeedRebuild();
         if (acceleratedCompositingForOverflowScrollEnabled())
             compositor()->setShouldReevaluateCompositingAfterLayout();
     }
 }
 
 void RenderLayer::dirtyStackingContainerZOrderLists()
 {
     RenderLayer* stackingContainer = this->ancestorStackingContainer();
     if (stackingContainer)
@@ skipping 118 matching lines @@
         for (RenderLayer* child = firstChild(); child; child = child->nextSibling()) {
             // Ignore reflections.
             if (!m_reflection || reflectionLayer() != child)
                 child->collectLayers(includeHiddenLayers, behavior, posBuffer, negBuffer, layerToForceAsStackingContainer);
         }
     }
 }
 
 void RenderLayer::updateLayerListsIfNeeded()
 {
-    bool shouldUpdateDescendantsAreContiguousInStackingOrder = acceleratedCompositingForOverflowScrollEnabled() && isStackingContext() && (m_zOrderListsDirty || m_normalFlowListDirty) && m_descendantsAreContiguousInStackingOrderDirty;
     updateZOrderLists();
     updateNormalFlowList();
 
     if (RenderLayer* reflectionLayer = this->reflectionLayer()) {
         reflectionLayer->updateZOrderLists();
         reflectionLayer->updateNormalFlowList();
     }
-
-    if (shouldUpdateDescendantsAreContiguousInStackingOrder) {
-        updateDescendantsAreContiguousInStackingOrder();
-        // The above function can cause us to update m_needsCompositedScrolling
-        // and dirty our layer lists. Refresh them if necessary.
-        updateZOrderLists();
-        updateNormalFlowList();
-    }
 }
 
 void RenderLayer::repaintIncludingDescendants()
 {
     renderer()->repaint();
     for (RenderLayer* curr = firstChild(); curr; curr = curr->nextSibling())
         curr->repaintIncludingDescendants();
 }
 
 void RenderLayer::setBackingNeedsRepaint()
@@ skipping 152 matching lines @@
         return true;
 
     if (hasVisibleBoxDecorations())
         return true;
 
     return false;
 }
 
 void RenderLayer::updateVisibilityAfterStyleChange(const RenderStyle* oldStyle)
 {
-    EVisibility oldVisibility = oldStyle ? oldStyle->visibility() : VISIBLE;
-    if (oldVisibility == renderer()->style()->visibility() || !renderer()->isOutOfFlowPositioned())
-        return;
-
-    if (renderer()->style()->visibility() == VISIBLE)
-        setAncestorChainHasOutOfFlowPositionedDescendant(renderer()->containingBlock());
-    else
-        dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
+    if (!oldStyle || (oldStyle->visibility() != renderer()->style()->visibility()))
+        compositor()->setNeedsUpdateCompositingRequirementsState();
 }
 
 void RenderLayer::updateStackingContextsAfterStyleChange(const RenderStyle* oldStyle)
 {
     bool wasStackingContext = oldStyle ? isStackingContext(oldStyle) : false;
     EVisibility oldVisibility = oldStyle ? oldStyle->visibility() : VISIBLE;
     int oldZIndex = oldStyle ? oldStyle->zIndex() : 0;
 
     // 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 && oldVisibility == renderer()->style()->visibility() && oldZIndex == renderer()->style()->zIndex())
         return;
 
     dirtyStackingContainerZOrderLists();
 
     if (isStackingContainer())
         dirtyZOrderLists();
     else
         clearZOrderLists();
 
-    updateNeedsCompositedScrolling();
+    compositor()->setNeedsUpdateCompositingRequirementsState();
 }
 
 static bool overflowRequiresScrollbar(EOverflow overflow)
 {
     return overflow == OSCROLL;
 }
 
 static bool overflowDefinesAutomaticScrollbar(EOverflow overflow)
 {
     return overflow == OAUTO || overflow == OOVERLAY;
@@ skipping 28 matching lines @@
 
     if (needsVerticalScrollbar && oldStyle && oldStyle->overflowY() == OSCROLL && overflowY != OSCROLL) {
         ASSERT(hasVerticalScrollbar());
         m_vBar->setEnabled(true);
     }
 
     if (!m_scrollDimensionsDirty)
         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)
 {
+    if (oldStyle && (renderer()->style()->position() == oldStyle->position()))
+        return;
+
     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 (!wasOutOfFlowPositioned && !isOutOfFlowPositioned)
+        return;
+
+    // Even if the layer remains out-of-flow, a change to this property
+    // will likely change its containing block. We must clear these bits
+    // so that they can be set properly by the RenderLayerCompositor.
+    for (RenderLayer* ancestor = parent(); ancestor; ancestor = ancestor->parent())
+        ancestor->m_hasUnclippedDescendant = false;
+
+    // Ensures that we reset the above bits correctly.
+    compositor()->setNeedsUpdateCompositingRequirementsState();
+
+    if (wasOutOfFlowPositioned && isOutOfFlowPositioned)
+        return;
+
+    if (isOutOfFlowPositioned) {
+        setAncestorChainHasOutOfFlowPositionedDescendant();
+        compositor()->addOutOfFlowPositionedLayer(this);
+    } else {
+        dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
+        compositor()->removeOutOfFlowPositionedLayer(this);
     }
 }
 
 static bool hasOrHadFilters(const RenderStyle* oldStyle, const RenderStyle* newStyle)
 {
     ASSERT(newStyle);
     return (oldStyle && oldStyle->hasFilter()) || newStyle->hasFilter();
 }
 
 inline bool RenderLayer::needsCompositingLayersRebuiltForClip(const RenderStyle* oldStyle, const RenderStyle* newStyle) const
@@ skipping 132 matching lines @@
 
     FrameView* frameView = frame->view();
     if (!frameView)
         return;
 
     bool isVisibleToHitTest = renderer()->visibleToHitTesting();
     if (HTMLFrameOwnerElement* owner = frame->ownerElement())
         isVisibleToHitTest &= owner->renderer() && owner->renderer()->visibleToHitTesting();
 
     bool updatedScrollableAreaSet = false;
-    if (hasOverflow && isVisibleToHitTest)
+    if (hasOverflow && isVisibleToHitTest) {
         updatedScrollableAreaSet = frameView->addScrollableArea(this);
-    else
+        compositor()->setNeedsUpdateCompositingRequirementsState();
+    } else {
         updatedScrollableAreaSet = frameView->removeScrollableArea(this);
-
-    if (updatedScrollableAreaSet)
-        updateNeedsCompositedScrolling();
+        setNeedsCompositedScrolling(false);
+    }
 }
 
 void RenderLayer::updateScrollCornerStyle()
 {
     RenderObject* actualRenderer = rendererForScrollbar(renderer());
     RefPtr<RenderStyle> corner = renderer()->hasOverflowClip() ? actualRenderer->getUncachedPseudoStyle(PseudoStyleRequest(SCROLLBAR_CORNER), actualRenderer->style()) : PassRefPtr<RenderStyle>(0);
     if (corner) {
         if (!m_scrollCorner) {
             m_scrollCorner = RenderScrollbarPart::createAnonymous(renderer()->document());
             m_scrollCorner->setParent(renderer());
@@ skipping 230 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif