Separate the geometry update from rebuilding the GraphicsLayer tree

Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp

Index: Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp
diff --git a/Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp b/Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..36ceb395013141983941ee1da92acbaf143f581d
--- /dev/null
+++ b/Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp
@@ -0,0 +1,525 @@
+/*
+ * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
+ * Copyright (C) 2014 Google Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE COMPUTER, INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "core/rendering/compositing/CompositingRequirementsUpdater.h"
+
+#include "CSSPropertyNames.h"
+#include "HTMLNames.h"
+#include "RuntimeEnabledFeatures.h"
+#include "core/animation/ActiveAnimations.h"
+#include "core/animation/DocumentAnimations.h"
+#include "core/dom/FullscreenElementStack.h"
+#include "core/dom/NodeList.h"
+#include "core/frame/DeprecatedScheduleStyleRecalcDuringCompositingUpdate.h"
+#include "core/frame/FrameView.h"
+#include "core/frame/LocalFrame.h"
+#include "core/frame/Settings.h"
+#include "core/html/HTMLCanvasElement.h"
+#include "core/html/HTMLIFrameElement.h"
+#include "core/html/HTMLMediaElement.h"
+#include "core/html/canvas/CanvasRenderingContext.h"
+#include "core/inspector/InspectorInstrumentation.h"
+#include "core/page/Chrome.h"
+#include "core/page/Page.h"
+#include "core/page/scrolling/ScrollingConstraints.h"
+#include "core/page/scrolling/ScrollingCoordinator.h"
+#include "core/rendering/HitTestResult.h"
+#include "core/rendering/RenderApplet.h"
+#include "core/rendering/RenderEmbeddedObject.h"
+#include "core/rendering/RenderFullScreen.h"
+#include "core/rendering/RenderIFrame.h"
+#include "core/rendering/RenderLayerStackingNode.h"
+#include "core/rendering/RenderLayerStackingNodeIterator.h"
+#include "core/rendering/RenderReplica.h"
+#include "core/rendering/RenderVideo.h"
+#include "core/rendering/RenderView.h"
+#include "core/rendering/compositing/CompositedLayerMapping.h"
+#include "core/rendering/compositing/GraphicsLayerUpdater.h"
+#include "core/rendering/compositing/RenderLayerCompositor.h"
+#include "platform/OverscrollTheme.h"
+#include "platform/TraceEvent.h"
+#include "platform/geometry/TransformState.h"
+#include "platform/graphics/GraphicsLayer.h"
+#include "platform/scroll/ScrollbarTheme.h"
+#include "public/platform/Platform.h"
+#include "wtf/TemporaryChange.h"
+
+namespace WebCore {
+
+class OverlapMapContainer {
+public:
+    void add(const IntRect& bounds)
+    {
+        m_layerRects.append(bounds);
+        m_boundingBox.unite(bounds);
+    }
+
+    bool overlapsLayers(const IntRect& bounds) const
+    {
+        // Checking with the bounding box will quickly reject cases when
+        // layers are created for lists of items going in one direction and
+        // never overlap with each other.
+        if (!bounds.intersects(m_boundingBox))
+            return false;
+        for (unsigned i = 0; i < m_layerRects.size(); i++) {
+            if (m_layerRects[i].intersects(bounds))
+                return true;
+        }
+        return false;
+    }
+
+    void unite(const OverlapMapContainer& otherContainer)
+    {
+        m_layerRects.appendVector(otherContainer.m_layerRects);
+        m_boundingBox.unite(otherContainer.m_boundingBox);
+    }
+private:
+    Vector<IntRect, 64> m_layerRects;
+    IntRect m_boundingBox;
+};
+
+class CompositingRequirementsUpdater::OverlapMap {
+    WTF_MAKE_NONCOPYABLE(OverlapMap);
+public:
+    OverlapMap()
+    {
+        // Begin by assuming the root layer will be composited so that there
+        // is something on the stack. The root layer should also never get a
+        // finishCurrentOverlapTestingContext() call.
+        beginNewOverlapTestingContext();
+    }
+
+    void add(RenderLayer* layer, const IntRect& bounds)
+    {
+        if (layer->isRootLayer() || bounds.isEmpty())
+            return;
+
+        // Layers do not contribute to overlap immediately--instead, they will
+        // contribute to overlap as soon as they have been recursively processed
+        // and popped off the stack.
+        ASSERT(m_overlapStack.size() >= 2);
+        m_overlapStack[m_overlapStack.size() - 2].add(bounds);
+    }
+
+    bool overlapsLayers(const IntRect& bounds) const
+    {
+        return m_overlapStack.last().overlapsLayers(bounds);
+    }
+
+    void beginNewOverlapTestingContext()
+    {
+        // This effectively creates a new "clean slate" for overlap state.
+        // This is used when we know that a subtree or remaining set of
+        // siblings does not need to check overlap with things behind it.
+        m_overlapStack.append(OverlapMapContainer());
+    }
+
+    void finishCurrentOverlapTestingContext()
+    {
+        // The overlap information on the top of the stack is still necessary
+        // for checking overlap of any layers outside this context that may
+        // overlap things from inside this context. Therefore, we must merge
+        // the information from the top of the stack before popping the stack.
+        //
+        // FIXME: we may be able to avoid this deep copy by rearranging how
+        //        overlapMap state is managed.
+        m_overlapStack[m_overlapStack.size() - 2].unite(m_overlapStack.last());
+        m_overlapStack.removeLast();
+    }
+
+private:
+    Vector<OverlapMapContainer> m_overlapStack;
+};
+
+class CompositingRequirementsUpdater::RecursionData {
+public:
+    RecursionData(RenderLayer* compAncestor, RenderLayer* mostRecentCompositedLayer, bool testOverlap)
+        : m_compositingAncestor(compAncestor)
+        , m_mostRecentCompositedLayer(mostRecentCompositedLayer)
+        , m_subtreeIsCompositing(false)
+        , m_hasUnisolatedCompositedBlendingDescendant(false)
+        , m_testingOverlap(testOverlap)
+#ifndef NDEBUG
+        , m_depth(0)
+#endif
+    {
+    }
+
+    RecursionData(const RecursionData& other)
+        : m_compositingAncestor(other.m_compositingAncestor)
+        , m_mostRecentCompositedLayer(other.m_mostRecentCompositedLayer)
+        , m_subtreeIsCompositing(other.m_subtreeIsCompositing)
+        , m_hasUnisolatedCompositedBlendingDescendant(other.m_hasUnisolatedCompositedBlendingDescendant)
+        , m_testingOverlap(other.m_testingOverlap)
+#ifndef NDEBUG
+        , m_depth(other.m_depth + 1)
+#endif
+    {
+    }
+
+    RenderLayer* m_compositingAncestor;
+    RenderLayer* m_mostRecentCompositedLayer; // in paint order regardless of hierarchy.
+    bool m_subtreeIsCompositing;
+    bool m_hasUnisolatedCompositedBlendingDescendant;
+    bool m_testingOverlap;
+#ifndef NDEBUG
+    int m_depth;
+#endif
+};
+
+static bool requiresCompositingOrSquashing(CompositingReasons reasons)
+{
+#ifndef NDEBUG
+    bool fastAnswer = reasons != CompositingReasonNone;
+    bool slowAnswer = requiresCompositing(reasons) || requiresSquashing(reasons);
+    ASSERT(fastAnswer == slowAnswer);
+#endif
+    return reasons != CompositingReasonNone;
+}
+
+static CompositingReasons subtreeReasonsForCompositing(RenderObject* renderer, bool hasCompositedDescendants, bool has3DTransformedDescendants)
+{
+    CompositingReasons subtreeReasons = CompositingReasonNone;
+
+    // FIXME: this seems to be a potentially different layer than the layer for which this was called. May not be an error, but is very confusing.
+    RenderLayer* layer = toRenderBoxModelObject(renderer)->layer();
+
+    // When a layer has composited descendants, some effects, like 2d transforms, filters, masks etc must be implemented
+    // via compositing so that they also apply to those composited descdendants.
+    if (hasCompositedDescendants) {
+        if (layer->transform())
+            subtreeReasons |= CompositingReasonTransformWithCompositedDescendants;
+
+        if (layer->shouldIsolateCompositedDescendants()) {
+            ASSERT(layer->stackingNode()->isStackingContext());
+            subtreeReasons |= CompositingReasonIsolateCompositedDescendants;
+        }
+
+        // If the implementation of createsGroup changes, we need to be aware of that in this part of code.
+        ASSERT((renderer->isTransparent() || renderer->hasMask() || renderer->hasFilter() || renderer->hasBlendMode()) == renderer->createsGroup());
+        if (renderer->isTransparent())
+            subtreeReasons |= CompositingReasonOpacityWithCompositedDescendants;
+        if (renderer->hasMask())
+            subtreeReasons |= CompositingReasonMaskWithCompositedDescendants;
+        if (renderer->hasFilter())
+            subtreeReasons |= CompositingReasonFilterWithCompositedDescendants;
+        if (renderer->hasBlendMode())
+            subtreeReasons |= CompositingReasonBlendingWithCompositedDescendants;
+
+        if (renderer->hasReflection())
+            subtreeReasons |= CompositingReasonReflectionWithCompositedDescendants;
+
+        if (renderer->hasClipOrOverflowClip())
+            subtreeReasons |= CompositingReasonClipsCompositingDescendants;
+    }
+
+
+    // A layer with preserve-3d or perspective only needs to be composited if there are descendant layers that
+    // will be affected by the preserve-3d or perspective.
+    if (has3DTransformedDescendants) {
+        if (renderer->style()->transformStyle3D() == TransformStyle3DPreserve3D)
+            subtreeReasons |= CompositingReasonPreserve3DWith3DDescendants;
+
+        if (renderer->style()->hasPerspective())
+            subtreeReasons |= CompositingReasonPerspectiveWith3DDescendants;
+    }
+
+    return subtreeReasons;
+}
+
+CompositingRequirementsUpdater::CompositingRequirementsUpdater(RenderView& renderView, CompositingReasonFinder& compositingReasonFinder, bool* needsToRecomputeCompositingRequirements)
+    : m_renderView(renderView)
+    , m_compositingReasonFinder(compositingReasonFinder)
+    , m_needsToRecomputeCompositingRequirements(needsToRecomputeCompositingRequirements)
+{
+}
+
+CompositingRequirementsUpdater::~CompositingRequirementsUpdater()
+{
+}
+
+void CompositingRequirementsUpdater::update(RenderLayer* root)
+{
+    TRACE_EVENT0("blink_rendering", "CompositingRequirementsUpdater::updateRecursive");
+
+    // Go through the layers in presentation order, so that we can compute which RenderLayers need compositing layers.
+    // FIXME: we could maybe do this and the hierarchy udpate in one pass, but the parenting logic would be more complex.
+    RecursionData recursionData(root, 0, true);
+    OverlapMap overlapTestRequestMap;
+    bool saw3DTransform = false;
+
+    // FIXME: Passing these unclippedDescendants down and keeping track
+    // of them dynamically, we are requiring a full tree walk. This
+    // should be removed as soon as proper overlap testing based on
+    // scrolling and animation bounds is implemented (crbug.com/252472).
+    Vector<RenderLayer*> unclippedDescendants;
+    IntRect absoluteDecendantBoundingBox;
+    updateRecursive(0, root, overlapTestRequestMap, recursionData, saw3DTransform, unclippedDescendants, absoluteDecendantBoundingBox);
+}
+
+void CompositingRequirementsUpdater::updateRecursive(RenderLayer* ancestorLayer, RenderLayer* layer, OverlapMap& overlapMap, RecursionData& currentRecursionData, bool& descendantHas3DTransform, Vector<RenderLayer*>& unclippedDescendants, IntRect& absoluteDecendantBoundingBox)
+{
+    RenderLayerCompositor* compositor = m_renderView.compositor();
+
+    layer->stackingNode()->updateLayerListsIfNeeded();
+
+    // Clear the flag
+    layer->setHasCompositingDescendant(false);
+
+    // Start by assuming this layer will not need to composite.
+    CompositingReasons reasonsToComposite = CompositingReasonNone;
+
+    // First accumulate the straightforward compositing reasons.
+    CompositingReasons directReasons = m_compositingReasonFinder.directReasons(layer, m_needsToRecomputeCompositingRequirements);
+
+    // Video is special. It's the only RenderLayer type that can both have
+    // RenderLayer children and whose children can't use its backing to render
+    // into. These children (the controls) always need to be promoted into their
+    // own layers to draw on top of the accelerated video.
+    if (currentRecursionData.m_compositingAncestor && currentRecursionData.m_compositingAncestor->renderer()->isVideo())
+        directReasons |= CompositingReasonVideoOverlay;
+
+    if (compositor->canBeComposited(layer))
+        reasonsToComposite |= directReasons;
+
+    // Next, accumulate reasons related to overlap.
+    // If overlap testing is used, this reason will be overridden. If overlap testing is not
+    // used, we must assume we overlap if there is anything composited behind us in paint-order.
+    CompositingReasons overlapCompositingReason = currentRecursionData.m_subtreeIsCompositing ? CompositingReasonAssumedOverlap : CompositingReasonNone;
+
+    if (m_renderView.compositorDrivenAcceleratedScrollingEnabled()) {
+        Vector<size_t> unclippedDescendantsToRemove;
+        for (size_t i = 0; i < unclippedDescendants.size(); i++) {
+            RenderLayer* unclippedDescendant = unclippedDescendants.at(i);
+            // If we've reached the containing block of one of the unclipped
+            // descendants, that element is no longer relevant to whether or not we
+            // should opt in. Unfortunately we can't easily remove from the list
+            // while we're iterating, so we have to store it for later removal.
+            if (unclippedDescendant->renderer()->containingBlock() == layer->renderer()) {
+                unclippedDescendantsToRemove.append(i);
+                continue;
+            }
+            if (layer->scrollsWithRespectTo(unclippedDescendant))
+                reasonsToComposite |= CompositingReasonAssumedOverlap;
+        }
+
+        // Remove irrelevant unclipped descendants in reverse order so our stored
+        // indices remain valid.
+        for (size_t i = 0; i < unclippedDescendantsToRemove.size(); i++)
+            unclippedDescendants.remove(unclippedDescendantsToRemove.at(unclippedDescendantsToRemove.size() - i - 1));
+
+        if (reasonsToComposite & CompositingReasonOutOfFlowClipping)
+            unclippedDescendants.append(layer);
+    }
+
+    const IntRect& absBounds = layer->ancestorDependentProperties().clippedAbsoluteBoundingBox;
+    absoluteDecendantBoundingBox = absBounds;
+
+    if (currentRecursionData.m_testingOverlap && !requiresCompositingOrSquashing(directReasons))
+        overlapCompositingReason = overlapMap.overlapsLayers(absBounds) ? CompositingReasonOverlap : CompositingReasonNone;
+
+    reasonsToComposite |= overlapCompositingReason;
+
+    // The children of this layer don't need to composite, unless there is
+    // a compositing layer among them, so start by inheriting the compositing
+    // ancestor with m_subtreeIsCompositing set to false.
+    RecursionData childRecursionData(currentRecursionData);
+    childRecursionData.m_subtreeIsCompositing = false;
+
+    bool willBeCompositedOrSquashed = compositor->canBeComposited(layer) && requiresCompositingOrSquashing(reasonsToComposite);
+    if (willBeCompositedOrSquashed) {
+        // Tell the parent it has compositing descendants.
+        currentRecursionData.m_subtreeIsCompositing = true;
+        // This layer now acts as the ancestor for kids.
+        childRecursionData.m_compositingAncestor = layer;
+
+        // Here we know that all children and the layer's own contents can blindly paint into
+        // this layer's backing, until a descendant is composited. So, we don't need to check
+        // for overlap with anything behind this layer.
+        overlapMap.beginNewOverlapTestingContext();
+        // This layer is going to be composited, so children can safely ignore the fact that there's an
+        // animation running behind this layer, meaning they can rely on the overlap map testing again.
+        childRecursionData.m_testingOverlap = true;
+    }
+
+#if !ASSERT_DISABLED
+    LayerListMutationDetector mutationChecker(layer->stackingNode());
+#endif
+
+    bool anyDescendantHas3DTransform = false;
+    bool willHaveForegroundLayer = false;
+
+    if (layer->stackingNode()->isStackingContainer()) {
+        RenderLayerStackingNodeIterator iterator(*layer->stackingNode(), NegativeZOrderChildren);
+        while (RenderLayerStackingNode* curNode = iterator.next()) {
+            IntRect absoluteChildDecendantBoundingBox;
+            updateRecursive(layer, curNode->layer(), overlapMap, childRecursionData, anyDescendantHas3DTransform, unclippedDescendants, absoluteChildDecendantBoundingBox);
+            absoluteDecendantBoundingBox.unite(absoluteChildDecendantBoundingBox);
+
+            // If we have to make a layer for this child, make one now so we can have a contents layer
+            // (since we need to ensure that the -ve z-order child renders underneath our contents).
+            if (childRecursionData.m_subtreeIsCompositing) {
+                reasonsToComposite |= CompositingReasonNegativeZIndexChildren;
+
+                if (!willBeCompositedOrSquashed) {
+                    // make layer compositing
+                    childRecursionData.m_compositingAncestor = layer;
+                    overlapMap.beginNewOverlapTestingContext();
+                    willBeCompositedOrSquashed = true;
+                    willHaveForegroundLayer = true;
+
+                    // FIXME: temporary solution for the first negative z-index composited child:
+                    //        re-compute the absBounds for the child so that we can add the
+                    //        negative z-index child's bounds to the new overlap context.
+                    overlapMap.beginNewOverlapTestingContext();
+                    overlapMap.add(curNode->layer(), curNode->layer()->ancestorDependentProperties().clippedAbsoluteBoundingBox);
+                    overlapMap.finishCurrentOverlapTestingContext();
+                }
+            }
+        }
+    }
+
+    if (willHaveForegroundLayer) {
+        ASSERT(willBeCompositedOrSquashed);
+        // A foreground layer effectively is a new backing for all subsequent children, so
+        // we don't need to test for overlap with anything behind this. So, we can finish
+        // the previous context that was accumulating rects for the negative z-index
+        // children, and start with a fresh new empty context.
+        overlapMap.finishCurrentOverlapTestingContext();
+        overlapMap.beginNewOverlapTestingContext();
+        // This layer is going to be composited, so children can safely ignore the fact that there's an
+        // animation running behind this layer, meaning they can rely on the overlap map testing again
+        childRecursionData.m_testingOverlap = true;
+    }
+
+    if (requiresCompositing(reasonsToComposite)) {
+        currentRecursionData.m_mostRecentCompositedLayer = layer;
+        childRecursionData.m_mostRecentCompositedLayer = layer;
+    }
+
+    RenderLayerStackingNodeIterator iterator(*layer->stackingNode(), NormalFlowChildren | PositiveZOrderChildren);
+    while (RenderLayerStackingNode* curNode = iterator.next()) {
+        IntRect absoluteChildDecendantBoundingBox;
+        updateRecursive(layer, curNode->layer(), overlapMap, childRecursionData, anyDescendantHas3DTransform, unclippedDescendants, absoluteChildDecendantBoundingBox);
+        absoluteDecendantBoundingBox.unite(absoluteChildDecendantBoundingBox);
+    }
+
+    currentRecursionData.m_mostRecentCompositedLayer = childRecursionData.m_mostRecentCompositedLayer;
+
+    // Now that the subtree has been traversed, we can check for compositing reasons that depended on the state of the subtree.
+
+    // If we entered compositing mode during the recursion, the root will also need to be composited (as long as accelerated compositing is enabled).
+    if (layer->isRootLayer()) {
+        // FIXME: How can we be in compositing mode but not have accelerated compositing?
+        if (compositor->inCompositingMode() && compositor->hasAcceleratedCompositing())
+            willBeCompositedOrSquashed = true;
+    }
+
+    // All layers (even ones that aren't being composited) need to get added to
+    // the overlap map. Layers that are not separately composited will paint into their
+    // compositing ancestor's backing, and so are still considered for overlap.
+    if (childRecursionData.m_compositingAncestor && !childRecursionData.m_compositingAncestor->isRootLayer())
+        overlapMap.add(layer, absBounds);
+
+    if (layer->stackingNode()->isStackingContext()) {
+        layer->setShouldIsolateCompositedDescendants(childRecursionData.m_hasUnisolatedCompositedBlendingDescendant);
+    } else {
+        layer->setShouldIsolateCompositedDescendants(false);
+        currentRecursionData.m_hasUnisolatedCompositedBlendingDescendant = childRecursionData.m_hasUnisolatedCompositedBlendingDescendant;
+    }
+
+    // Now check for reasons to become composited that depend on the state of descendant layers.
+    CompositingReasons subtreeCompositingReasons = subtreeReasonsForCompositing(layer->renderer(), childRecursionData.m_subtreeIsCompositing, anyDescendantHas3DTransform);
+    reasonsToComposite |= subtreeCompositingReasons;
+    if (!willBeCompositedOrSquashed && compositor->canBeComposited(layer) && requiresCompositingOrSquashing(subtreeCompositingReasons)) {
+        childRecursionData.m_compositingAncestor = layer;
+        // FIXME: this context push is effectively a no-op but needs to exist for
+        // now, because the code is designed to push overlap information to the
+        // second-from-top context of the stack.
+        overlapMap.beginNewOverlapTestingContext();
+        overlapMap.add(layer, absoluteDecendantBoundingBox);
+        willBeCompositedOrSquashed = true;
+    }
+
+    // If the original layer is composited, the reflection needs to be, too.
+    if (layer->reflectionInfo()) {
+        // FIXME: Shouldn't we call computeCompositingRequirements to handle a reflection overlapping with another renderer?
+        RenderLayer* reflectionLayer = layer->reflectionInfo()->reflectionLayer();
+        CompositingReasons reflectionCompositingReason = willBeCompositedOrSquashed ? CompositingReasonReflectionOfCompositedParent : CompositingReasonNone;
+        reflectionLayer->setCompositingReasons(reflectionLayer->compositingReasons() | reflectionCompositingReason);
+    }
+
+    // Subsequent layers in the parent's stacking context may also need to composite.
+    if (childRecursionData.m_subtreeIsCompositing)
+        currentRecursionData.m_subtreeIsCompositing = true;
+
+    if (willBeCompositedOrSquashed && layer->blendInfo().hasBlendMode())
+        currentRecursionData.m_hasUnisolatedCompositedBlendingDescendant = true;
+
+    // Set the flag to say that this SC has compositing children.
+    layer->setHasCompositingDescendant(childRecursionData.m_subtreeIsCompositing);
+
+    // Turn overlap testing off for later layers if it's already off, or if we have an animating transform.
+    // Note that if the layer clips its descendants, there's no reason to propagate the child animation to the parent layers. That's because
+    // we know for sure the animation is contained inside the clipping rectangle, which is already added to the overlap map.
+    bool isCompositedClippingLayer = compositor->canBeComposited(layer) && (reasonsToComposite & CompositingReasonClipsCompositingDescendants);
+    if ((!childRecursionData.m_testingOverlap && !isCompositedClippingLayer) || isRunningAcceleratedTransformAnimation(layer->renderer()))
+        currentRecursionData.m_testingOverlap = false;
+
+    if (childRecursionData.m_compositingAncestor == layer && !layer->isRootLayer())
+        overlapMap.finishCurrentOverlapTestingContext();
+
+    if (layer->isRootLayer()) {
+        // The root layer needs to be composited if anything else in the tree is composited.
+        // Otherwise, we can disable compositing entirely.
+        if (childRecursionData.m_subtreeIsCompositing || requiresCompositingOrSquashing(reasonsToComposite) || compositor->inForcedCompositingMode()) {
+            willBeCompositedOrSquashed = true;
+            reasonsToComposite |= CompositingReasonRoot;
+        } else {
+            compositor->enableCompositingMode(false);
+            willBeCompositedOrSquashed = false;
+            reasonsToComposite = CompositingReasonNone;
+        }
+    }
+
+    if (requiresCompositing(reasonsToComposite))
+        currentRecursionData.m_mostRecentCompositedLayer = layer;
+
+    // At this point we have finished collecting all reasons to composite this layer.
+    layer->setCompositingReasons(reasonsToComposite);
+
+    descendantHas3DTransform |= anyDescendantHas3DTransform || layer->has3DTransform();
+}
+
+bool CompositingRequirementsUpdater::isRunningAcceleratedTransformAnimation(RenderObject* renderer) const
+{
+    if (!m_compositingReasonFinder.hasAnimationTrigger())
+        return false;
+    return renderer->style()->hasCurrentTransformAnimation();
+}
+
+} // namespace WebCore