ASSERT(isSelfPaintingLayer() || hasSelfPaintingLayerDescendant()) on nytimes.com...
[WebKit-https.git] / Source / WebCore / rendering / RenderLayerCompositor.cpp
index 3defc8a..790791b 100644 (file)
@@ -35,6 +35,7 @@
 #include "DocumentTimeline.h"
 #include "Frame.h"
 #include "FrameView.h"
+#include "FullscreenManager.h"
 #include "GraphicsLayer.h"
 #include "HTMLCanvasElement.h"
 #include "HTMLIFrameElement.h"
 #include "Settings.h"
 #include "TiledBacking.h"
 #include "TransformState.h"
+#include <wtf/HexNumber.h>
 #include <wtf/MemoryPressureHandler.h>
 #include <wtf/SetForScope.h>
 #include <wtf/text/CString.h>
 #include <wtf/text/StringBuilder.h>
+#include <wtf/text/StringConcatenateNumbers.h>
 #include <wtf/text/TextStream.h>
 
 #if PLATFORM(IOS_FAMILY)
@@ -104,6 +107,11 @@ static const Seconds throttledLayerFlushDelay { 500_ms };
 
 using namespace HTMLNames;
 
+struct ScrollingTreeState {
+    Optional<ScrollingNodeID> parentNodeID;
+    size_t nextChildIndex { 0 };
+};
+
 class OverlapMapContainer {
 public:
     void add(const LayoutRect& bounds)
@@ -224,23 +232,43 @@ struct RenderLayerCompositor::CompositingState {
     {
     }
     
-    CompositingState(const CompositingState& other)
-        : compositingAncestor(other.compositingAncestor)
-        , subtreeIsCompositing(other.subtreeIsCompositing)
-        , testingOverlap(other.testingOverlap)
-        , fullPaintOrderTraversalRequired(other.fullPaintOrderTraversalRequired)
-        , descendantsRequireCompositingUpdate(other.descendantsRequireCompositingUpdate)
-        , ancestorHasTransformAnimation(other.ancestorHasTransformAnimation)
+    CompositingState stateForPaintOrderChildren(RenderLayer& layer) const
+    {
+        UNUSED_PARAM(layer);
+        CompositingState childState(compositingAncestor);
+        if (layer.isStackingContext())
+            childState.stackingContextAncestor = &layer;
+        else
+            childState.stackingContextAncestor = stackingContextAncestor;
+
+        childState.subtreeIsCompositing = false;
+        childState.testingOverlap = testingOverlap;
+        childState.fullPaintOrderTraversalRequired = fullPaintOrderTraversalRequired;
+        childState.descendantsRequireCompositingUpdate = descendantsRequireCompositingUpdate;
+        childState.ancestorHasTransformAnimation = ancestorHasTransformAnimation;
 #if ENABLE(CSS_COMPOSITING)
-        , hasNotIsolatedCompositedBlendingDescendants(other.hasNotIsolatedCompositedBlendingDescendants)
+        childState.hasNotIsolatedCompositedBlendingDescendants = false; // FIXME: should this only be reset for stacking contexts?
 #endif
 #if ENABLE(TREE_DEBUGGING)
-        , depth(other.depth + 1)
+        childState.depth = depth + 1;
 #endif
+        return childState;
+    }
+
+    void propagateStateFromChildren(const CompositingState& childState)
     {
+        // Subsequent layers in the parent stacking context also need to composite.
+        subtreeIsCompositing |= childState.subtreeIsCompositing;
+        fullPaintOrderTraversalRequired |= childState.fullPaintOrderTraversalRequired;
     }
-    
+
+    void propagateStateFromChildrenForUnchangedSubtree(const CompositingState& childState)
+    {
+        subtreeIsCompositing |= childState.subtreeIsCompositing;
+    }
+
     RenderLayer* compositingAncestor;
+    RenderLayer* stackingContextAncestor { nullptr };
     bool subtreeIsCompositing { false };
     bool testingOverlap { true };
     bool fullPaintOrderTraversalRequired { false };
@@ -254,6 +282,22 @@ struct RenderLayerCompositor::CompositingState {
 #endif
 };
 
+struct RenderLayerCompositor::BackingSharingState {
+    RenderLayer* backingProviderCandidate { nullptr };
+    RenderLayer* backingProviderStackingContext { nullptr };
+    Vector<WeakPtr<RenderLayer>> backingSharingLayers;
+
+    void resetBackingProviderCandidate(RenderLayer* candidateLayer = nullptr, RenderLayer* candidateStackingContext = nullptr)
+    {
+        if (!backingSharingLayers.isEmpty()) {
+            ASSERT(backingProviderCandidate);
+            backingProviderCandidate->backing()->setBackingSharingLayers(WTFMove(backingSharingLayers));
+        }
+        backingProviderCandidate = candidateLayer;
+        backingProviderStackingContext = candidateLayer ? candidateStackingContext : nullptr;
+    }
+};
+
 struct RenderLayerCompositor::OverlapExtent {
     LayoutRect bounds;
     bool extentComputed { false };
@@ -266,7 +310,12 @@ struct RenderLayerCompositor::OverlapExtent {
 #if !LOG_DISABLED
 static inline bool compositingLogEnabled()
 {
-    return LogCompositing.state == WTFLogChannelOn;
+    return LogCompositing.state == WTFLogChannelState::On;
+}
+
+static inline bool layersLogEnabled()
+{
+    return LogLayers.state == WTFLogChannelState::On;
 }
 #endif
 
@@ -284,8 +333,27 @@ RenderLayerCompositor::RenderLayerCompositor(RenderView& renderView)
 RenderLayerCompositor::~RenderLayerCompositor()
 {
     // Take care that the owned GraphicsLayers are deleted first as their destructors may call back here.
-    m_clipLayer = nullptr;
-    m_scrollLayer = nullptr;
+    GraphicsLayer::unparentAndClear(m_rootContentsLayer);
+    
+    GraphicsLayer::unparentAndClear(m_clipLayer);
+    GraphicsLayer::unparentAndClear(m_scrollContainerLayer);
+    GraphicsLayer::unparentAndClear(m_scrolledContentsLayer);
+
+    GraphicsLayer::unparentAndClear(m_overflowControlsHostLayer);
+
+    GraphicsLayer::unparentAndClear(m_layerForHorizontalScrollbar);
+    GraphicsLayer::unparentAndClear(m_layerForVerticalScrollbar);
+    GraphicsLayer::unparentAndClear(m_layerForScrollCorner);
+
+#if ENABLE(RUBBER_BANDING)
+    GraphicsLayer::unparentAndClear(m_layerForOverhangAreas);
+    GraphicsLayer::unparentAndClear(m_contentShadowLayer);
+    GraphicsLayer::unparentAndClear(m_layerForTopOverhangArea);
+    GraphicsLayer::unparentAndClear(m_layerForBottomOverhangArea);
+    GraphicsLayer::unparentAndClear(m_layerForHeader);
+    GraphicsLayer::unparentAndClear(m_layerForFooter);
+#endif
+
     ASSERT(m_rootLayerAttachment == RootLayerUnattached);
 }
 
@@ -405,7 +473,7 @@ bool RenderLayerCompositor::didRecalcStyleWithNoPendingLayout()
 
 void RenderLayerCompositor::customPositionForVisibleRectComputation(const GraphicsLayer* graphicsLayer, FloatPoint& position) const
 {
-    if (graphicsLayer != m_scrollLayer.get())
+    if (graphicsLayer != m_scrolledContentsLayer.get())
         return;
 
     FloatPoint scrollPosition = -position;
@@ -421,12 +489,6 @@ void RenderLayerCompositor::notifyFlushRequired(const GraphicsLayer* layer)
     scheduleLayerFlush(layer->canThrottleLayerFlush());
 }
 
-void RenderLayerCompositor::scheduleLayerFlushNow()
-{
-    m_hasPendingLayerFlush = false;
-    page().chrome().client().scheduleCompositingLayerFlush();
-}
-
 void RenderLayerCompositor::scheduleLayerFlush(bool canThrottle)
 {
     ASSERT(!m_flushingLayers);
@@ -434,11 +496,12 @@ void RenderLayerCompositor::scheduleLayerFlush(bool canThrottle)
     if (canThrottle)
         startInitialLayerFlushTimerIfNeeded();
 
-    if (canThrottle && isThrottlingLayerFlushes()) {
+    if (canThrottle && isThrottlingLayerFlushes())
         m_hasPendingLayerFlush = true;
-        return;
+    else {
+        m_hasPendingLayerFlush = false;
+        page().renderingUpdateScheduler().scheduleRenderingUpdate();
     }
-    scheduleLayerFlushNow();
 }
 
 FloatRect RenderLayerCompositor::visibleRectForLayerFlushing() const
@@ -447,8 +510,8 @@ FloatRect RenderLayerCompositor::visibleRectForLayerFlushing() const
 #if PLATFORM(IOS_FAMILY)
     return frameView.exposedContentRect();
 #else
-    // Having a m_scrollLayer indicates that we're doing scrolling via GraphicsLayers.
-    FloatRect visibleRect = m_scrollLayer ? FloatRect({ }, frameView.sizeForVisibleContent()) : frameView.visibleContentRect();
+    // Having a m_scrolledContentsLayer indicates that we're doing scrolling via GraphicsLayers.
+    FloatRect visibleRect = m_scrolledContentsLayer ? FloatRect({ }, frameView.sizeForVisibleContent()) : frameView.visibleContentRect();
 
     if (frameView.viewExposedRect())
         visibleRect.intersect(frameView.viewExposedRect().value());
@@ -465,7 +528,7 @@ void RenderLayerCompositor::flushPendingLayerChanges(bool isFlushRoot)
     // As long as we're not the root of the flush, we can bail.
     if (!isFlushRoot && rootLayerAttachment() == RootLayerAttachedViaEnclosingFrame)
         return;
-    
+
     if (rootLayerAttachment() == RootLayerUnattached) {
 #if PLATFORM(IOS_FAMILY)
         startLayerFlushTimerIfNeeded();
@@ -488,11 +551,18 @@ void RenderLayerCompositor::flushPendingLayerChanges(bool isFlushRoot)
         }
         
         ASSERT(m_flushingLayers);
+
+#if ENABLE(TREE_DEBUGGING)
+        if (layersLogEnabled()) {
+            LOG(Layers, "RenderLayerCompositor::flushPendingLayerChanges");
+            showGraphicsLayerTree(m_rootContentsLayer.get());
+        }
+#endif
     }
 
+#if PLATFORM(IOS_FAMILY)
     updateScrollCoordinatedLayersAfterFlushIncludingSubframes();
 
-#if PLATFORM(IOS_FAMILY)
     if (isFlushRoot)
         page().chrome().client().didFlushCompositingLayers();
 #endif
@@ -501,6 +571,7 @@ void RenderLayerCompositor::flushPendingLayerChanges(bool isFlushRoot)
     startLayerFlushTimerIfNeeded();
 }
 
+#if PLATFORM(IOS_FAMILY)
 void RenderLayerCompositor::updateScrollCoordinatedLayersAfterFlushIncludingSubframes()
 {
     updateScrollCoordinatedLayersAfterFlush();
@@ -517,38 +588,36 @@ void RenderLayerCompositor::updateScrollCoordinatedLayersAfterFlushIncludingSubf
 
 void RenderLayerCompositor::updateScrollCoordinatedLayersAfterFlush()
 {
-    updateCustomLayersAfterFlush();
-
-    HashSet<RenderLayer*> layersNeedingUpdate;
-    std::swap(m_scrollCoordinatedLayersNeedingUpdate, layersNeedingUpdate);
-    
-    for (auto* layer : layersNeedingUpdate)
-        updateScrollCoordinatedStatus(*layer, ScrollingNodeChangeFlags::Layer);
-}
-
-void RenderLayerCompositor::updateCustomLayersAfterFlush()
-{
-#if PLATFORM(IOS_FAMILY)
     if (m_legacyScrollingLayerCoordinator) {
-        m_legacyScrollingLayerCoordinator->registerAllViewportConstrainedLayers(*this, m_scrollCoordinatedLayers);
+        m_legacyScrollingLayerCoordinator->registerAllViewportConstrainedLayers(*this);
         m_legacyScrollingLayerCoordinator->registerScrollingLayersNeedingUpdate();
     }
-#endif
 }
+#endif
 
-void RenderLayerCompositor::didChangePlatformLayerForLayer(RenderLayer& layer, const GraphicsLayer* graphicsLayer)
+void RenderLayerCompositor::didChangePlatformLayerForLayer(RenderLayer& layer, const GraphicsLayer*)
 {
-    if (m_scrollCoordinatedLayers.contains(&layer))
-        m_scrollCoordinatedLayersNeedingUpdate.add(&layer);
-
 #if PLATFORM(IOS_FAMILY)
     if (m_legacyScrollingLayerCoordinator)
         m_legacyScrollingLayerCoordinator->didChangePlatformLayerForLayer(layer);
 #endif
 
+    auto* scrollingCoordinator = this->scrollingCoordinator();
+    if (!scrollingCoordinator)
+        return;
+
     auto* backing = layer.backing();
-    if (backing->backgroundLayerPaintsFixedRootBackground() && graphicsLayer == backing->backgroundLayer())
-        fixedRootBackgroundLayerChanged();
+    if (auto nodeID = backing->scrollingNodeIDForRole(ScrollCoordinationRole::Scrolling))
+        updateScrollingNodeLayers(nodeID, layer, *scrollingCoordinator);
+
+    if (auto nodeID = backing->scrollingNodeIDForRole(ScrollCoordinationRole::ViewportConstrained))
+        scrollingCoordinator->setNodeLayers(nodeID, { backing->graphicsLayer() });
+
+    if (auto nodeID = backing->scrollingNodeIDForRole(ScrollCoordinationRole::FrameHosting))
+        scrollingCoordinator->setNodeLayers(nodeID, { backing->graphicsLayer() });
+
+    if (auto nodeID = backing->scrollingNodeIDForRole(ScrollCoordinationRole::Positioning))
+        scrollingCoordinator->setNodeLayers(nodeID, { backing->graphicsLayer() });
 }
 
 void RenderLayerCompositor::didPaintBacking(RenderLayerBacking*)
@@ -569,7 +638,7 @@ void RenderLayerCompositor::didChangeVisibleRect()
     bool requiresFlush = rootLayer->visibleRectChangeRequiresFlush(visibleRect);
     LOG_WITH_STREAM(Compositing, stream << "RenderLayerCompositor::didChangeVisibleRect " << visibleRect << " requiresFlush " << requiresFlush);
     if (requiresFlush)
-        scheduleLayerFlushNow();
+        scheduleLayerFlush();
 }
 
 void RenderLayerCompositor::notifyFlushBeforeDisplayRefresh(const GraphicsLayer*)
@@ -614,6 +683,32 @@ void RenderLayerCompositor::cancelCompositingLayerUpdate()
     m_updateCompositingLayersTimer.stop();
 }
 
+static Optional<ScrollingNodeID> frameHostingNodeForFrame(Frame& frame)
+{
+    if (!frame.document() || !frame.view())
+        return { };
+
+    // Find the frame's enclosing layer in our render tree.
+    auto* ownerElement = frame.document()->ownerElement();
+    if (!ownerElement)
+        return { };
+
+    auto* frameRenderer = ownerElement->renderer();
+    if (!frameRenderer || !is<RenderWidget>(frameRenderer))
+        return { };
+
+    auto& widgetRenderer = downcast<RenderWidget>(*frameRenderer);
+    if (!widgetRenderer.hasLayer() || !widgetRenderer.layer()->isComposited()) {
+        LOG(Scrolling, "frameHostingNodeForFrame: frame renderer has no layer or is not composited.");
+        return { };
+    }
+
+    if (auto frameHostingNodeID = widgetRenderer.layer()->backing()->scrollingNodeIDForRole(ScrollCoordinationRole::FrameHosting))
+        return frameHostingNodeID;
+
+    return { };
+}
+
 // Returns true on a successful update.
 bool RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType updateType, RenderLayer* updateRoot)
 {
@@ -637,8 +732,10 @@ bool RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType update
 
     // Avoid updating the layers with old values. Compositing layers will be updated after the layout is finished.
     // This happens when m_updateCompositingLayersTimer fires before layout is updated.
-    if (m_renderView.needsLayout())
+    if (m_renderView.needsLayout()) {
+        LOG_WITH_STREAM(Compositing, stream << "RenderLayerCompositor " << this << " updateCompositingLayers " << updateType << " - m_renderView.needsLayout, bailing ");
         return false;
+    }
 
     if (!m_compositing && (m_forceCompositingMode || (isMainFrameCompositor() && page().pageOverlayController().overlayCount())))
         enableCompositingMode(true);
@@ -661,6 +758,11 @@ bool RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType update
         updateRoot->setDescendantsNeedCompositingRequirementsTraversal();
     }
 
+    if (updateType == CompositingUpdateType::AfterLayout) {
+        // Ensure that post-layout updates push new scroll position and viewport rects onto the root node.
+        rootRenderLayer().setNeedsScrollingTreeUpdate();
+    }
+
     if (!updateRoot->hasDescendantNeedingCompositingRequirementsTraversal() && !m_compositing) {
         LOG_WITH_STREAM(Compositing, stream << " no compositing work to do");
         return true;
@@ -671,7 +773,6 @@ bool RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType update
         return true;
     }
 
-    bool isFullUpdate = true;
     ++m_compositingUpdateCount;
 
     AnimationUpdateBlock animationUpdateBlock(&m_renderView.frameView().frame().animation());
@@ -700,10 +801,11 @@ bool RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType update
     // FIXME: optimize root-only update.
     if (updateRoot->hasDescendantNeedingCompositingRequirementsTraversal() || updateRoot->needsCompositingRequirementsTraversal()) {
         CompositingState compositingState(updateRoot);
+        BackingSharingState backingSharingState;
         OverlapMap overlapMap;
 
         bool descendantHas3DTransform = false;
-        computeCompositingRequirements(nullptr, rootRenderLayer(), overlapMap, compositingState, descendantHas3DTransform);
+        computeCompositingRequirements(nullptr, rootRenderLayer(), overlapMap, compositingState, backingSharingState, descendantHas3DTransform);
     }
 
     LOG(Compositing, "\nRenderLayerCompositor::updateCompositingLayers - mid");
@@ -713,21 +815,21 @@ bool RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType update
 #endif
 
     if (updateRoot->hasDescendantNeedingUpdateBackingOrHierarchyTraversal() || updateRoot->needsUpdateBackingOrHierarchyTraversal()) {
+        ScrollingTreeState scrollingTreeState = { 0, 0 };
+        if (!m_renderView.frame().isMainFrame())
+            scrollingTreeState.parentNodeID = frameHostingNodeForFrame(m_renderView.frame());
+
         Vector<Ref<GraphicsLayer>> childList;
-        updateBackingAndHierarchy(*updateRoot, childList);
+        updateBackingAndHierarchy(*updateRoot, childList, scrollingTreeState);
 
         // Host the document layer in the RenderView's root layer.
-        if (isFullUpdate) {
-            appendDocumentOverlayLayers(childList);
-            // Even when childList is empty, don't drop out of compositing mode if there are
-            // composited layers that we didn't hit in our traversal (e.g. because of visibility:hidden).
-            if (childList.isEmpty() && !needsCompositingForContentOrOverlays())
-                destroyRootLayer();
-            else if (m_rootContentLayer)
-                m_rootContentLayer->setChildren(WTFMove(childList));
-        }
-    
-        reattachSubframeScrollLayers();
+        appendDocumentOverlayLayers(childList);
+        // Even when childList is empty, don't drop out of compositing mode if there are
+        // composited layers that we didn't hit in our traversal (e.g. because of visibility:hidden).
+        if (childList.isEmpty() && !needsCompositingForContentOrOverlays())
+            destroyRootLayer();
+        else if (m_rootContentsLayer)
+            m_rootContentsLayer->setChildren(WTFMove(childList));
     }
 
 #if !LOG_DISABLED
@@ -748,8 +850,6 @@ bool RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType update
     if (compositingLogEnabled()) {
         LOG(Compositing, "RenderLayerCompositor::updateCompositingLayers - post");
         showPaintOrderTree(m_renderView.layer());
-        LOG(Compositing, "RenderLayerCompositor::updateCompositingLayers - GraphicsLayers post, contentLayersCount %d", m_contentLayersCount);
-        showGraphicsLayerTree(m_rootContentLayer.get());
     }
 #endif
 
@@ -758,15 +858,23 @@ bool RenderLayerCompositor::updateCompositingLayers(CompositingUpdateType update
     return true;
 }
 
-void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestorLayer, RenderLayer& layer, OverlapMap& overlapMap, CompositingState& compositingState, bool& descendantHas3DTransform)
+static bool backingProviderLayerCanIncludeLayer(const RenderLayer& sharedLayer, const RenderLayer& layer)
 {
-    if (!layer.hasDescendantNeedingCompositingRequirementsTraversal() && !layer.needsCompositingRequirementsTraversal() && !compositingState.fullPaintOrderTraversalRequired && !compositingState.descendantsRequireCompositingUpdate) {
-        traverseUnchangedSubtree(ancestorLayer, layer, overlapMap, compositingState, descendantHas3DTransform);
+    return layer.ancestorLayerIsInContainingBlockChain(sharedLayer);
+}
+
+void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestorLayer, RenderLayer& layer, OverlapMap& overlapMap, CompositingState& compositingState, BackingSharingState& backingSharingState, bool& descendantHas3DTransform)
+{
+    if (!layer.hasDescendantNeedingCompositingRequirementsTraversal()
+        && !layer.needsCompositingRequirementsTraversal()
+        && !compositingState.fullPaintOrderTraversalRequired
+        && !compositingState.descendantsRequireCompositingUpdate) {
+        traverseUnchangedSubtree(ancestorLayer, layer, overlapMap, compositingState, backingSharingState, descendantHas3DTransform);
         return;
     }
 
 #if ENABLE(TREE_DEBUGGING)
-    LOG(Compositing, "%*p computeCompositingRequirements", 12 + compositingState.depth * 2, &layer);
+    LOG(Compositing, "%*p %s computeCompositingRequirements (backing provider candidate %p)", 12 + compositingState.depth * 2, &layer, layer.isNormalFlowOnly() ? "n" : "s", backingSharingState.backingProviderCandidate);
 #endif
 
     // FIXME: maybe we can avoid updating all remaining layers in paint order.
@@ -777,6 +885,7 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
     layer.updateLayerListsIfNeeded();
 
     layer.setHasCompositingDescendant(false);
+    layer.setBackingProviderLayer(nullptr);
 
     // We updated compositing for direct reasons in layerStyleChanged(). Here, check for compositing that can only be evaluated after layout.
     RequiresCompositingData queryData;
@@ -798,12 +907,23 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
     overlapMap.geometryMap().pushMappingsToAncestor(&layer, ancestorLayer, respectTransforms);
 
     RenderLayer::IndirectCompositingReason compositingReason = compositingState.subtreeIsCompositing ? RenderLayer::IndirectCompositingReason::Stacking : RenderLayer::IndirectCompositingReason::None;
+    bool layerPaintsIntoProvidedBacking = false;
 
     // If we know for sure the layer is going to be composited, don't bother looking it up in the overlap map
     if (!willBeComposited && !overlapMap.isEmpty() && compositingState.testingOverlap) {
         computeExtent(overlapMap, layer, layerExtent);
+
         // If we're testing for overlap, we only need to composite if we overlap something that is already composited.
-        compositingReason = overlapMap.overlapsLayers(layerExtent.bounds) ? RenderLayer::IndirectCompositingReason::Overlap : RenderLayer::IndirectCompositingReason::None;
+        if (overlapMap.overlapsLayers(layerExtent.bounds)) {
+            if (backingSharingState.backingProviderCandidate && canBeComposited(layer) && backingProviderLayerCanIncludeLayer(*backingSharingState.backingProviderCandidate, layer)) {
+                backingSharingState.backingSharingLayers.append(makeWeakPtr(layer));
+                LOG(Compositing, " layer %p can share with %p", &layer, backingSharingState.backingProviderCandidate);
+                compositingReason = RenderLayer::IndirectCompositingReason::None;
+                layerPaintsIntoProvidedBacking = true;
+            } else
+                compositingReason = RenderLayer::IndirectCompositingReason::Overlap;
+        } else
+            compositingReason = RenderLayer::IndirectCompositingReason::None;
     }
 
 #if ENABLE(VIDEO)
@@ -825,27 +945,42 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
     // 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 subtreeIsCompositing set to false.
-    CompositingState childState(compositingState);
-    childState.subtreeIsCompositing = false;
-#if ENABLE(CSS_COMPOSITING)
-    childState.hasNotIsolatedCompositedBlendingDescendants = false;
-#endif
+    CompositingState childState = compositingState.stateForPaintOrderChildren(layer);
 
-    if (willBeComposited) {
-        // Tell the parent it has compositing descendants.
-        compositingState.subtreeIsCompositing = true;
+    auto layerWillComposite = [&](bool postDescendants = false) {
         // This layer now acts as the ancestor for kids.
         childState.compositingAncestor = &layer;
-
         overlapMap.pushCompositingContainer();
+        
+        if (postDescendants) {
+            childState.subtreeIsCompositing = true;
+            addToOverlapMapRecursive(overlapMap, layer);
+        }
+
         // 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.
         childState.testingOverlap = true;
+        willBeComposited = true;
+
+        layerPaintsIntoProvidedBacking = false;
+        layer.disconnectFromBackingProviderLayer();
+        backingSharingState.backingSharingLayers.removeAll(&layer);
+    };
+
+    if (willBeComposited) {
+        // Tell the parent it has compositing descendants.
+        compositingState.subtreeIsCompositing = true;
+        
+        layerWillComposite();
 
         computeExtent(overlapMap, layer, layerExtent);
         childState.ancestorHasTransformAnimation |= layerExtent.hasTransformAnimation;
         // Too hard to compute animated bounds if both us and some ancestor is animating transform.
         layerExtent.animationCausesExtentUncertainty |= layerExtent.hasTransformAnimation && compositingState.ancestorHasTransformAnimation;
+
+        // Compositing for any reason disables backing sharing.
+        LOG_WITH_STREAM(Compositing, stream << &layer << " is compositing - flushing sharing to " << backingSharingState.backingProviderCandidate << " with " << backingSharingState.backingSharingLayers.size() << " sharing layers");
+        backingSharingState.resetBackingProviderCandidate();
     }
 
 #if !ASSERT_DISABLED
@@ -855,27 +990,22 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
     bool anyDescendantHas3DTransform = false;
 
     for (auto* childLayer : layer.negativeZOrderLayers()) {
-        computeCompositingRequirements(&layer, *childLayer, overlapMap, childState, anyDescendantHas3DTransform);
+        computeCompositingRequirements(&layer, *childLayer, overlapMap, childState, backingSharingState, anyDescendantHas3DTransform);
 
         // 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 (!willBeComposited && childState.subtreeIsCompositing) {
             // make layer compositing
             layer.setIndirectCompositingReason(RenderLayer::IndirectCompositingReason::BackgroundLayer);
-            childState.compositingAncestor = &layer;
-            overlapMap.pushCompositingContainer();
-            // 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
-            childState.testingOverlap = true;
-            willBeComposited = true;
+            layerWillComposite();
         }
     }
     
     for (auto* childLayer : layer.normalFlowLayers())
-        computeCompositingRequirements(&layer, *childLayer, overlapMap, childState, anyDescendantHas3DTransform);
+        computeCompositingRequirements(&layer, *childLayer, overlapMap, childState, backingSharingState, anyDescendantHas3DTransform);
 
     for (auto* childLayer : layer.positiveZOrderLayers())
-        computeCompositingRequirements(&layer, *childLayer, overlapMap, childState, anyDescendantHas3DTransform);
+        computeCompositingRequirements(&layer, *childLayer, overlapMap, childState, backingSharingState, anyDescendantHas3DTransform);
 
     // If we just entered compositing mode, the root will have become composited (as long as accelerated compositing is enabled).
     if (layer.isRenderViewLayer()) {
@@ -892,18 +1022,21 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
         addToOverlapMap(overlapMap, layer, layerExtent);
 
 #if ENABLE(CSS_COMPOSITING)
+    bool isolatedCompositedBlending = layer.isolatesCompositedBlending();
     layer.setHasNotIsolatedCompositedBlendingDescendants(childState.hasNotIsolatedCompositedBlendingDescendants);
+    if (layer.isolatesCompositedBlending() != isolatedCompositedBlending) {
+        // isolatedCompositedBlending affects the result of clippedByAncestor().
+        layer.setChildrenNeedCompositingGeometryUpdate();
+    }
+
     ASSERT(!layer.hasNotIsolatedCompositedBlendingDescendants() || layer.hasNotIsolatedBlendingDescendants());
 #endif
     // Now check for reasons to become composited that depend on the state of descendant layers.
     RenderLayer::IndirectCompositingReason indirectCompositingReason;
     if (!willBeComposited && canBeComposited(layer)
-        && requiresCompositingForIndirectReason(layer.renderer(), childState.subtreeIsCompositing, anyDescendantHas3DTransform, indirectCompositingReason)) {
+        && requiresCompositingForIndirectReason(layer, compositingState.compositingAncestor, childState.subtreeIsCompositing, anyDescendantHas3DTransform, layerPaintsIntoProvidedBacking, indirectCompositingReason)) {
         layer.setIndirectCompositingReason(indirectCompositingReason);
-        childState.compositingAncestor = &layer;
-        overlapMap.pushCompositingContainer();
-        addToOverlapMapRecursive(overlapMap, layer);
-        willBeComposited = true;
+        layerWillComposite(true);
     }
     
     if (layer.reflectionLayer()) {
@@ -911,10 +1044,6 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
         layer.reflectionLayer()->setIndirectCompositingReason(willBeComposited ? RenderLayer::IndirectCompositingReason::Stacking : RenderLayer::IndirectCompositingReason::None);
     }
 
-    // Subsequent layers in the parent stacking context also need to composite.
-    compositingState.subtreeIsCompositing |= childState.subtreeIsCompositing;
-    compositingState.fullPaintOrderTraversalRequired |= childState.fullPaintOrderTraversalRequired;
-
     // Set the flag to say that this layer has compositing children.
     layer.setHasCompositingDescendant(childState.subtreeIsCompositing);
 
@@ -927,18 +1056,11 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
     if ((!childState.testingOverlap && !isCompositedClippingLayer) || layerExtent.knownToBeHaveExtentUncertainty())
         compositingState.testingOverlap = false;
     
-    if (isCompositedClippingLayer) {
-        if (!willBeComposited) {
-            childState.compositingAncestor = &layer;
-            overlapMap.pushCompositingContainer();
-            addToOverlapMapRecursive(overlapMap, layer);
-            willBeComposited = true;
-        }
-    }
+    if (isCompositedClippingLayer & !willBeComposited)
+        layerWillComposite(true);
 
 #if ENABLE(CSS_COMPOSITING)
-    if ((willBeComposited && layer.hasBlendMode())
-        || (layer.hasNotIsolatedCompositedBlendingDescendants() && !layer.isolatesCompositedBlending()))
+    if ((willBeComposited && layer.hasBlendMode()) || (layer.hasNotIsolatedCompositedBlendingDescendants() && !layer.isolatesCompositedBlending()))
         compositingState.hasNotIsolatedCompositedBlendingDescendants = true;
 #endif
 
@@ -956,7 +1078,9 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
         willBeComposited = false;
 #endif
     }
-    
+
+    compositingState.propagateStateFromChildren(childState);
+
     ASSERT(willBeComposited == needsToBeComposited(layer, queryData));
 
     // Create or destroy backing here. However, we can't update geometry because layers above us may become composited
@@ -967,6 +1091,16 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
         layer.setChildrenNeedCompositingGeometryUpdate();
         // The composited bounds of enclosing layers depends on which descendants are composited, so they need a geometry update.
         layer.setNeedsCompositingGeometryUpdateOnAncestors();
+    } else if (layer.isComposited())
+        layer.backing()->clearBackingSharingLayers();
+
+    if (backingSharingState.backingProviderCandidate && &layer == backingSharingState.backingProviderStackingContext) {
+        LOG_WITH_STREAM(Compositing, stream << &layer << " popping stacking context " << backingSharingState.backingProviderStackingContext << ", flushing candidate " << backingSharingState.backingProviderCandidate << " with " << backingSharingState.backingSharingLayers.size() << " sharing layers");
+        backingSharingState.resetBackingProviderCandidate();
+    } else if (!backingSharingState.backingProviderCandidate && layer.isComposited()) {
+        LOG_WITH_STREAM(Compositing, stream << &layer << " compositing - sharing candidate " << backingSharingState.backingProviderCandidate << " with " << backingSharingState.backingSharingLayers.size() << " sharing layers");
+        // Flush out any earlier candidate in this stacking context. This layer becomes a candidate.
+        backingSharingState.resetBackingProviderCandidate(&layer, compositingState.stackingContextAncestor);
     }
 
     if (layer.reflectionLayer() && updateLayerCompositingState(*layer.reflectionLayer(), queryData, CompositingChangeRepaintNow))
@@ -981,7 +1115,7 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
     }
 
 #if ENABLE(TREE_DEBUGGING)
-    LOG(Compositing, "%*p computeCompositingRequirements - willBeComposited %d", 12 + compositingState.depth * 2, &layer, willBeComposited);
+    LOG(Compositing, "%*p computeCompositingRequirements - willBeComposited %d (backing provider candidate %p)", 12 + compositingState.depth * 2, &layer, willBeComposited, backingSharingState.backingProviderCandidate);
 #endif
 
     layer.clearCompositingRequirementsTraversalState();
@@ -990,7 +1124,7 @@ void RenderLayerCompositor::computeCompositingRequirements(RenderLayer* ancestor
 }
 
 // We have to traverse unchanged layers to fill in the overlap map.
-void RenderLayerCompositor::traverseUnchangedSubtree(RenderLayer* ancestorLayer, RenderLayer& layer, OverlapMap& overlapMap, CompositingState& compositingState, bool& descendantHas3DTransform)
+void RenderLayerCompositor::traverseUnchangedSubtree(RenderLayer* ancestorLayer, RenderLayer& layer, OverlapMap& overlapMap, CompositingState& compositingState, BackingSharingState& backingSharingState, bool& descendantHas3DTransform)
 {
     ASSERT(!compositingState.fullPaintOrderTraversalRequired);
     ASSERT(!layer.hasDescendantNeedingCompositingRequirementsTraversal());
@@ -1016,11 +1150,13 @@ void RenderLayerCompositor::traverseUnchangedSubtree(RenderLayer* ancestorLayer,
     if (!layerIsComposited && !overlapMap.isEmpty() && compositingState.testingOverlap)
         computeExtent(overlapMap, layer, layerExtent);
 
-    CompositingState childState(compositingState);
-    childState.subtreeIsCompositing = false;
-#if ENABLE(CSS_COMPOSITING)
-    childState.hasNotIsolatedCompositedBlendingDescendants = false;
-#endif
+    if (layer.paintsIntoProvidedBacking()) {
+        ASSERT(backingSharingState.backingProviderCandidate);
+        ASSERT(backingProviderLayerCanIncludeLayer(*backingSharingState.backingProviderCandidate, layer));
+        backingSharingState.backingSharingLayers.append(makeWeakPtr(layer));
+    }
+
+    CompositingState childState = compositingState.stateForPaintOrderChildren(layer);
 
     if (layerIsComposited) {
         // Tell the parent it has compositing descendants.
@@ -1037,6 +1173,10 @@ void RenderLayerCompositor::traverseUnchangedSubtree(RenderLayer* ancestorLayer,
         childState.ancestorHasTransformAnimation |= layerExtent.hasTransformAnimation;
         // Too hard to compute animated bounds if both us and some ancestor is animating transform.
         layerExtent.animationCausesExtentUncertainty |= layerExtent.hasTransformAnimation && compositingState.ancestorHasTransformAnimation;
+
+        // Compositing for any reason disables backing sharing.
+        LOG_WITH_STREAM(Compositing, stream << "tus: " << &layer << " is compositing - flushing sharing to " << backingSharingState.backingProviderCandidate << " with " << backingSharingState.backingSharingLayers.size() << " sharing layers");
+        backingSharingState.resetBackingProviderCandidate();
     }
 
 #if !ASSERT_DISABLED
@@ -1046,16 +1186,16 @@ void RenderLayerCompositor::traverseUnchangedSubtree(RenderLayer* ancestorLayer,
     bool anyDescendantHas3DTransform = false;
 
     for (auto* childLayer : layer.negativeZOrderLayers()) {
-        traverseUnchangedSubtree(&layer, *childLayer, overlapMap, childState, anyDescendantHas3DTransform);
+        traverseUnchangedSubtree(&layer, *childLayer, overlapMap, childState, backingSharingState, anyDescendantHas3DTransform);
         if (childState.subtreeIsCompositing)
             ASSERT(layerIsComposited);
     }
     
     for (auto* childLayer : layer.normalFlowLayers())
-        traverseUnchangedSubtree(&layer, *childLayer, overlapMap, childState, anyDescendantHas3DTransform);
+        traverseUnchangedSubtree(&layer, *childLayer, overlapMap, childState, backingSharingState, anyDescendantHas3DTransform);
 
     for (auto* childLayer : layer.positiveZOrderLayers())
-        traverseUnchangedSubtree(&layer, *childLayer, overlapMap, childState, anyDescendantHas3DTransform);
+        traverseUnchangedSubtree(&layer, *childLayer, overlapMap, childState, backingSharingState, anyDescendantHas3DTransform);
 
     // All layers (even ones that aren't being composited) need to get added to
     // the overlap map. Layers that do not composite will draw into their
@@ -1065,9 +1205,7 @@ void RenderLayerCompositor::traverseUnchangedSubtree(RenderLayer* ancestorLayer,
     if (childState.compositingAncestor && !childState.compositingAncestor->isRenderViewLayer())
         addToOverlapMap(overlapMap, layer, layerExtent);
 
-    // Subsequent layers in the parent stacking context also need to composite.
-    if (childState.subtreeIsCompositing)
-        compositingState.subtreeIsCompositing = true;
+    compositingState.propagateStateFromChildrenForUnchangedSubtree(childState);
 
     // Set the flag to say that this layer has compositing children.
     ASSERT(layer.hasCompositingDescendant() == childState.subtreeIsCompositing);
@@ -1093,6 +1231,18 @@ void RenderLayerCompositor::traverseUnchangedSubtree(RenderLayer* ancestorLayer,
     if (childState.compositingAncestor == &layer && !layer.isRenderViewLayer())
         overlapMap.popCompositingContainer();
 
+    if (layer.isComposited())
+        layer.backing()->clearBackingSharingLayers();
+
+    if (backingSharingState.backingProviderCandidate && &layer == backingSharingState.backingProviderStackingContext) {
+        LOG_WITH_STREAM(Compositing, stream << &layer << " tus: popping stacking context " << backingSharingState.backingProviderStackingContext << ", flushing candidate " << backingSharingState.backingProviderCandidate << " with " << backingSharingState.backingSharingLayers.size() << " sharing layers");
+        backingSharingState.resetBackingProviderCandidate();
+    } else if (!backingSharingState.backingProviderCandidate && layer.isComposited()) {
+        LOG_WITH_STREAM(Compositing, stream << &layer << " tus: compositing - sharing candidate " << backingSharingState.backingProviderCandidate << " with " << backingSharingState.backingSharingLayers.size() << " sharing layers");
+        // Flush out any earlier candidate in this stacking context. This layer becomes a candidate.
+        backingSharingState.resetBackingProviderCandidate(&layer, compositingState.stackingContextAncestor);
+    }
+
     descendantHas3DTransform |= anyDescendantHas3DTransform || layer.has3DTransform();
 
     ASSERT(!layer.needsCompositingRequirementsTraversal());
@@ -1100,7 +1250,7 @@ void RenderLayerCompositor::traverseUnchangedSubtree(RenderLayer* ancestorLayer,
     overlapMap.geometryMap().popMappingsToAncestor(ancestorLayer);
 }
 
-void RenderLayerCompositor::updateBackingAndHierarchy(RenderLayer& layer, Vector<Ref<GraphicsLayer>>& childLayersOfEnclosingLayer, OptionSet<UpdateLevel> updateLevel, int depth)
+void RenderLayerCompositor::updateBackingAndHierarchy(RenderLayer& layer, Vector<Ref<GraphicsLayer>>& childLayersOfEnclosingLayer, ScrollingTreeState& scrollingTreeState, OptionSet<UpdateLevel> updateLevel, int depth)
 {
     layer.updateDescendantDependentFlags();
     layer.updateLayerListsIfNeeded();
@@ -1109,6 +1259,8 @@ void RenderLayerCompositor::updateBackingAndHierarchy(RenderLayer& layer, Vector
     if (layer.descendantsNeedUpdateBackingAndHierarchyTraversal())
         updateLevel.add(UpdateLevel::AllDescendants);
 
+    ScrollingTreeState stateForDescendants = scrollingTreeState;
+
     auto* layerBacking = layer.backing();
     if (layerBacking) {
         updateLevel.remove(UpdateLevel::CompositedChildren);
@@ -1130,8 +1282,12 @@ void RenderLayerCompositor::updateBackingAndHierarchy(RenderLayer& layer, Vector
             layerBacking->updateDebugIndicators(m_showDebugBorders, m_showRepaintCounter);
         }
         
-        if (layerNeedsUpdate || layer.needsCompositingGeometryUpdate() || layer.needsScrollingTreeUpdate())
+        OptionSet<ScrollingNodeChangeFlags> scrollingNodeChanges = { ScrollingNodeChangeFlags::Layer };
+        if (layerNeedsUpdate || layer.needsCompositingGeometryUpdate()) {
             layerBacking->updateGeometry();
+            scrollingNodeChanges.add(ScrollingNodeChangeFlags::LayerGeometry);
+        } else if (layer.needsScrollingTreeUpdate())
+            scrollingNodeChanges.add(ScrollingNodeChangeFlags::LayerGeometry);
 
         if (auto* reflection = layer.reflectionLayer()) {
             if (auto* reflectionBacking = reflection->backing()) {
@@ -1144,6 +1300,11 @@ void RenderLayerCompositor::updateBackingAndHierarchy(RenderLayer& layer, Vector
         if (!layer.parent())
             updateRootLayerPosition();
 
+        // FIXME: do based on dirty flags. Need to do this for changes of geometry, configuration and hierarchy.
+        // Need to be careful to do the right thing when a scroll-coordinated layer loses a scroll-coordinated ancestor.
+        stateForDescendants.parentNodeID = updateScrollCoordinationForLayer(layer, scrollingTreeState, layerBacking->coordinatedScrollingRoles(), scrollingNodeChanges);
+        stateForDescendants.nextChildIndex = 0;
+
 #if !LOG_DISABLED
         logLayerInfo(layer, "updateBackingAndHierarchy", depth);
 #else
@@ -1161,7 +1322,7 @@ void RenderLayerCompositor::updateBackingAndHierarchy(RenderLayer& layer, Vector
 
     bool requireDescendantTraversal = layer.hasDescendantNeedingUpdateBackingOrHierarchyTraversal()
         || (layer.hasCompositingDescendant() && (!layerBacking || layer.needsCompositingLayerConnection() || !updateLevel.isEmpty()));
-    
+
     bool requiresChildRebuild = layerBacking && layer.needsCompositingLayerConnection() && !layer.hasCompositingDescendant();
 
 #if !ASSERT_DISABLED
@@ -1178,15 +1339,15 @@ void RenderLayerCompositor::updateBackingAndHierarchy(RenderLayer& layer, Vector
 
     if (requireDescendantTraversal) {
         for (auto* renderLayer : layer.negativeZOrderLayers())
-            updateBackingAndHierarchy(*renderLayer, childList, updateLevel, depth + 1);
+            updateBackingAndHierarchy(*renderLayer, childList, stateForDescendants, updateLevel, depth + 1);
 
         appendForegroundLayerIfNecessary();
 
         for (auto* renderLayer : layer.normalFlowLayers())
-            updateBackingAndHierarchy(*renderLayer, childList, updateLevel, depth + 1);
+            updateBackingAndHierarchy(*renderLayer, childList, stateForDescendants, updateLevel, depth + 1);
         
         for (auto* renderLayer : layer.positiveZOrderLayers())
-            updateBackingAndHierarchy(*renderLayer, childList, updateLevel, depth + 1);
+            updateBackingAndHierarchy(*renderLayer, childList, stateForDescendants, updateLevel, depth + 1);
     } else if (requiresChildRebuild)
         appendForegroundLayerIfNecessary();
 
@@ -1226,6 +1387,9 @@ void RenderLayerCompositor::appendDocumentOverlayLayers(Vector<Ref<GraphicsLayer
     if (!isMainFrameCompositor() || !m_compositing)
         return;
 
+    if (!page().pageOverlayController().hasDocumentOverlays())
+        return;
+
     Ref<GraphicsLayer> overlayHost = page().pageOverlayController().layerWithDocumentOverlays();
     childList.append(WTFMove(overlayHost));
 }
@@ -1273,12 +1437,10 @@ void RenderLayerCompositor::logLayerInfo(const RenderLayer& layer, const char* p
     absoluteBounds.move(layer.offsetFromAncestor(m_renderView.layer()));
     
     StringBuilder logString;
-    logString.append(String::format("%*p id %" PRIu64 " (%.3f,%.3f-%.3f,%.3f) %.2fKB", 12 + depth * 2, &layer, backing->graphicsLayer()->primaryLayerID(),
-        absoluteBounds.x().toFloat(), absoluteBounds.y().toFloat(), absoluteBounds.maxX().toFloat(), absoluteBounds.maxY().toFloat(),
-        backing->backingStoreMemoryEstimate() / 1024));
-    
+    logString.append(makeString(pad(' ', 12 + depth * 2, hex(reinterpret_cast<uintptr_t>(&layer))), " id ", backing->graphicsLayer()->primaryLayerID(), " (", FormattedNumber::fixedWidth(absoluteBounds.x().toFloat(), 3), ',', FormattedNumber::fixedWidth(absoluteBounds.y().toFloat(), 3), '-', FormattedNumber::fixedWidth(absoluteBounds.maxX().toFloat(), 3), ',', FormattedNumber::fixedWidth(absoluteBounds.maxY().toFloat(), 3), ") ", FormattedNumber::fixedWidth(backing->backingStoreMemoryEstimate() / 1024, 2), "KB"));
+
     if (!layer.renderer().style().hasAutoZIndex())
-        logString.append(String::format(" z-index: %d", layer.renderer().style().zIndex()));
+        logString.append(makeString(" z-index: ", layer.renderer().style().zIndex()));
 
     logString.appendLiteral(" (");
     logString.append(logReasonsForCompositing(layer));
@@ -1343,6 +1505,20 @@ static bool styleAffectsLayerGeometry(const RenderStyle& style)
     return style.hasClip() || style.clipPath() || style.hasBorderRadius();
 }
 
+static bool recompositeChangeRequiresGeometryUpdate(const RenderStyle& oldStyle, const RenderStyle& newStyle)
+{
+    return oldStyle.transform() != newStyle.transform()
+        || oldStyle.transformOriginX() != newStyle.transformOriginX()
+        || oldStyle.transformOriginY() != newStyle.transformOriginY()
+        || oldStyle.transformOriginZ() != newStyle.transformOriginZ()
+        || oldStyle.transformStyle3D() != newStyle.transformStyle3D()
+        || oldStyle.perspective() != newStyle.perspective()
+        || oldStyle.perspectiveOriginX() != newStyle.perspectiveOriginX()
+        || oldStyle.perspectiveOriginY() != newStyle.perspectiveOriginY()
+        || oldStyle.backfaceVisibility() != newStyle.backfaceVisibility()
+        || !arePointingToEqualData(oldStyle.clipPath(), newStyle.clipPath());
+}
+
 void RenderLayerCompositor::layerStyleChanged(StyleDifference diff, RenderLayer& layer, const RenderStyle* oldStyle)
 {
     if (diff == StyleDifference::Equal)
@@ -1396,12 +1572,10 @@ void RenderLayerCompositor::layerStyleChanged(StyleDifference diff, RenderLayer&
     if (diff == StyleDifference::RecompositeLayer && layer.isComposited() && is<RenderWidget>(layer.renderer()))
         layer.setNeedsCompositingConfigurationUpdate();
 
-    if (diff >= StyleDifference::RecompositeLayer && oldStyle) {
-        if (oldStyle->transform() != newStyle.transform()) {
-            // FIXME: transform changes really need to trigger layout. See RenderElement::adjustStyleDifference().
-            layer.setNeedsPostLayoutCompositingUpdate();
-            layer.setNeedsCompositingGeometryUpdate();
-        }
+    if (diff >= StyleDifference::RecompositeLayer && oldStyle && recompositeChangeRequiresGeometryUpdate(*oldStyle, newStyle)) {
+        // FIXME: transform changes really need to trigger layout. See RenderElement::adjustStyleDifference().
+        layer.setNeedsPostLayoutCompositingUpdate();
+        layer.setNeedsCompositingGeometryUpdate();
     }
 
     if (diff >= StyleDifference::Layout) {
@@ -1473,7 +1647,7 @@ static RenderLayerModelObject& rendererForCompositingTests(const RenderLayer& la
 
 void RenderLayerCompositor::updateRootContentLayerClipping()
 {
-    m_rootContentLayer->setMasksToBounds(!m_renderView.settings().backgroundShouldExtendBeyondPage());
+    m_rootContentsLayer->setMasksToBounds(!m_renderView.settings().backgroundShouldExtendBeyondPage());
 }
 
 bool RenderLayerCompositor::updateBacking(RenderLayer& layer, RequiresCompositingData& queryData, CompositingChangeRepaint shouldRepaint, BackingRequired backingRequired)
@@ -1487,6 +1661,8 @@ bool RenderLayerCompositor::updateBacking(RenderLayer& layer, RequiresCompositin
     }
 
     if (backingRequired == BackingRequired::Yes) {
+        layer.disconnectFromBackingProviderLayer();
+
         enableCompositingMode();
         
         if (!layer.backing()) {
@@ -1497,17 +1673,17 @@ bool RenderLayerCompositor::updateBacking(RenderLayer& layer, RequiresCompositin
             layer.ensureBacking();
 
             if (layer.isRenderViewLayer() && useCoordinatedScrollingForLayer(layer)) {
-                updateScrollCoordinatedStatus(layer, { ScrollingNodeChangeFlags::Layer, ScrollingNodeChangeFlags::LayerGeometry });
+                auto& frameView = m_renderView.frameView();
                 if (auto* scrollingCoordinator = this->scrollingCoordinator())
-                    scrollingCoordinator->frameViewRootLayerDidChange(m_renderView.frameView());
+                    scrollingCoordinator->frameViewRootLayerDidChange(frameView);
 #if ENABLE(RUBBER_BANDING)
-                updateLayerForHeader(page().headerHeight());
-                updateLayerForFooter(page().footerHeight());
+                updateLayerForHeader(frameView.headerHeight());
+                updateLayerForFooter(frameView.footerHeight());
 #endif
                 updateRootContentLayerClipping();
 
                 if (auto* tiledBacking = layer.backing()->tiledBacking())
-                    tiledBacking->setTopContentInset(m_renderView.frameView().topContentInset());
+                    tiledBacking->setTopContentInset(frameView.topContentInset());
             }
 
             // This layer and all of its descendants have cached repaints rects that are relative to
@@ -1534,8 +1710,6 @@ bool RenderLayerCompositor::updateBacking(RenderLayer& layer, RequiresCompositin
                 }
             }
 
-            removeFromScrollCoordinatedLayers(layer);
-
             layer.clearBacking();
             layerChanged = true;
 
@@ -1641,13 +1815,19 @@ void RenderLayerCompositor::layerWasAdded(RenderLayer&, RenderLayer&)
 
 void RenderLayerCompositor::layerWillBeRemoved(RenderLayer& parent, RenderLayer& child)
 {
-    if (!child.isComposited() || parent.renderer().renderTreeBeingDestroyed())
+    if (parent.renderer().renderTreeBeingDestroyed())
         return;
 
-    removeFromScrollCoordinatedLayers(child);
-    repaintInCompositedAncestor(child, child.backing()->compositedBounds()); // FIXME: do via dirty bits?
+    if (child.isComposited())
+        repaintInCompositedAncestor(child, child.backing()->compositedBounds()); // FIXME: do via dirty bits?
+    else if (child.paintsIntoProvidedBacking()) {
+        auto* backingProviderLayer = child.backingProviderLayer();
+        // FIXME: Optimize this repaint.
+        backingProviderLayer->setBackingNeedsRepaint();
+        backingProviderLayer->backing()->removeBackingSharingLayer(child);
+    } else
+        return;
 
-    setCompositingParent(child, nullptr);
     child.setNeedsCompositingLayerConnection();
 }
 
@@ -1738,33 +1918,6 @@ void RenderLayerCompositor::addToOverlapMapRecursive(OverlapMap& overlapMap, con
         overlapMap.geometryMap().popMappingsToAncestor(ancestorLayer);
 }
 
-void RenderLayerCompositor::setCompositingParent(RenderLayer& childLayer, RenderLayer* parentLayer)
-{
-    ASSERT(!parentLayer || childLayer.ancestorCompositingLayer() == parentLayer);
-    ASSERT(childLayer.isComposited());
-
-    // It's possible to be called with a parent that isn't yet composited when we're doing
-    // partial updates as required by painting or hit testing. Just bail in that case;
-    // we'll do a full layer update soon.
-    if (!parentLayer || !parentLayer->isComposited())
-        return;
-
-    if (parentLayer) {
-        auto* hostingLayer = parentLayer->backing()->parentForSublayers();
-        auto* hostedLayer = childLayer.backing()->childForSuperlayers();
-        
-        hostingLayer->addChild(*hostedLayer);
-    } else
-        childLayer.backing()->childForSuperlayers()->removeFromParent();
-}
-
-void RenderLayerCompositor::removeCompositedChildren(RenderLayer& layer)
-{
-    ASSERT(layer.isComposited());
-
-    layer.backing()->parentForSublayers()->removeAllChildren();
-}
-
 #if ENABLE(VIDEO)
 bool RenderLayerCompositor::canAccelerateVideoRendering(RenderVideo& video) const
 {
@@ -1786,7 +1939,7 @@ void RenderLayerCompositor::frameViewDidChangeSize()
     if (auto* layer = m_renderView.layer())
         layer->setNeedsCompositingGeometryUpdate();
 
-    if (m_scrollLayer) {
+    if (m_scrolledContentsLayer) {
         updateScrollLayerClipping();
         frameViewDidScroll();
         updateOverflowControlsLayers();
@@ -1809,12 +1962,13 @@ bool RenderLayerCompositor::hasCoordinatedScrolling() const
 
 void RenderLayerCompositor::updateScrollLayerPosition()
 {
-    ASSERT(m_scrollLayer);
+    ASSERT(!hasCoordinatedScrolling());
+    ASSERT(m_scrolledContentsLayer);
 
     auto& frameView = m_renderView.frameView();
     IntPoint scrollPosition = frameView.scrollPosition();
 
-    m_scrollLayer->setPosition(FloatPoint(-scrollPosition.x(), -scrollPosition.y()));
+    m_scrolledContentsLayer->setPosition(FloatPoint(-scrollPosition.x(), -scrollPosition.y()));
 
     if (auto* fixedBackgroundLayer = fixedRootBackgroundLayer())
         fixedBackgroundLayer->setPosition(frameView.scrollPositionForFixedPosition());
@@ -1841,14 +1995,14 @@ FloatPoint RenderLayerCompositor::positionForClipLayer() const
 
 void RenderLayerCompositor::frameViewDidScroll()
 {
-    if (!m_scrollLayer)
+    if (!m_scrolledContentsLayer)
         return;
 
     // If there's a scrolling coordinator that manages scrolling for this frame view,
     // it will also manage updating the scroll layer position.
     if (hasCoordinatedScrolling()) {
         // We have to schedule a flush in order for the main TiledBacking to update its tile coverage.
-        scheduleLayerFlushNow();
+        scheduleLayerFlush();
         return;
     }
 
@@ -1875,23 +2029,15 @@ void RenderLayerCompositor::rootLayerConfigurationChanged()
     }
 }
 
-void RenderLayerCompositor::fixedRootBackgroundLayerChanged()
-{
-    if (m_renderView.renderTreeBeingDestroyed())
-        return;
-
-    if (m_renderView.layer()->isComposited())
-        updateScrollCoordinatedStatus(*m_renderView.layer(), ScrollingNodeChangeFlags::Layer);
-}
-
 String RenderLayerCompositor::layerTreeAsText(LayerTreeFlags flags)
 {
     updateCompositingLayers(CompositingUpdateType::AfterLayout);
 
-    if (!m_rootContentLayer)
+    if (!m_rootContentsLayer)
         return String();
 
     flushPendingLayerChanges(true);
+    page().renderingUpdateScheduler().scheduleCompositingLayerFlush();
 
     LayerTreeAsTextBehavior layerTreeBehavior = LayerTreeAsTextBehaviorNormal;
     if (flags & LayerTreeFlagsIncludeDebugInfo)
@@ -1910,14 +2056,18 @@ String RenderLayerCompositor::layerTreeAsText(LayerTreeFlags flags)
         layerTreeBehavior |= LayerTreeAsTextIncludeAcceleratesDrawing;
     if (flags & LayerTreeFlagsIncludeBackingStoreAttached)
         layerTreeBehavior |= LayerTreeAsTextIncludeBackingStoreAttached;
+    if (flags & LayerTreeFlagsIncludeRootLayerProperties)
+        layerTreeBehavior |= LayerTreeAsTextIncludeRootLayerProperties;
+    if (flags & LayerTreeFlagsIncludeEventRegion)
+        layerTreeBehavior |= LayerTreeAsTextIncludeEventRegion;
 
     // We skip dumping the scroll and clip layers to keep layerTreeAsText output
     // similar between platforms.
-    String layerTreeText = m_rootContentLayer->layerTreeAsText(layerTreeBehavior);
+    String layerTreeText = m_rootContentsLayer->layerTreeAsText(layerTreeBehavior);
 
     // Dump an empty layer tree only if the only composited layer is the main frame's tiled backing,
     // so that tests expecting us to drop out of accelerated compositing when there are no layers succeed.
-    if (!hasContentCompositingLayers() && documentUsesTiledBacking() && !(layerTreeBehavior & LayerTreeAsTextIncludeTileCaches))
+    if (!hasContentCompositingLayers() && documentUsesTiledBacking() && !(layerTreeBehavior & LayerTreeAsTextIncludeTileCaches) && !(layerTreeBehavior & LayerTreeAsTextIncludeRootLayerProperties))
         layerTreeText = emptyString();
 
     // The true root layer is not included in the dump, so if we want to report
@@ -1964,7 +2114,7 @@ bool RenderLayerCompositor::parentFrameContentLayers(RenderWidget& renderer)
 
     if (auto frameHostingNodeID = backing->scrollingNodeIDForRole(ScrollCoordinationRole::FrameHosting)) {
         auto* contentsRenderView = frameContentsRenderView(renderer);
-        if (auto frameRootScrollingNodeID = contentsRenderView->frameView().scrollLayerID()) {
+        if (auto frameRootScrollingNodeID = contentsRenderView->frameView().scrollingNodeID()) {
             if (auto* scrollingCoordinator = this->scrollingCoordinator())
                 scrollingCoordinator->insertNode(ScrollingNodeType::Subframe, frameRootScrollingNodeID, frameHostingNodeID, 0);
         }
@@ -1981,6 +2131,8 @@ void RenderLayerCompositor::repaintCompositedLayers()
 
 void RenderLayerCompositor::recursiveRepaintLayer(RenderLayer& layer)
 {
+    layer.updateLayerListsIfNeeded();
+
     // FIXME: This method does not work correctly with transforms.
     if (layer.isComposited() && !layer.backing()->paintsIntoCompositedAncestor())
         layer.setBackingNeedsRepaint();
@@ -2010,7 +2162,7 @@ GraphicsLayer* RenderLayerCompositor::rootGraphicsLayer() const
 {
     if (m_overflowControlsHostLayer)
         return m_overflowControlsHostLayer.get();
-    return m_rootContentLayer.get();
+    return m_rootContentsLayer.get();
 }
 
 void RenderLayerCompositor::setIsInWindow(bool isInWindow)
@@ -2034,7 +2186,7 @@ void RenderLayerCompositor::setIsInWindow(bool isInWindow)
         attachRootLayer(attachment);
 #if PLATFORM(IOS_FAMILY)
         if (m_legacyScrollingLayerCoordinator) {
-            m_legacyScrollingLayerCoordinator->registerAllViewportConstrainedLayers(*this, m_scrollCoordinatedLayers);
+            m_legacyScrollingLayerCoordinator->registerAllViewportConstrainedLayers(*this);
             m_legacyScrollingLayerCoordinator->registerAllScrollingLayers();
         }
 #endif
@@ -2054,10 +2206,8 @@ void RenderLayerCompositor::setIsInWindow(bool isInWindow)
 
 void RenderLayerCompositor::clearBackingForLayerIncludingDescendants(RenderLayer& layer)
 {
-    if (layer.isComposited()) {
-        removeFromScrollCoordinatedLayers(layer);
+    if (layer.isComposited())
         layer.clearBacking();
-    }
 
     for (auto* childLayer = layer.firstChild(); childLayer; childLayer = childLayer->nextSibling())
         clearBackingForLayerIncludingDescendants(*childLayer);
@@ -2070,18 +2220,18 @@ void RenderLayerCompositor::clearBackingForAllLayers()
 
 void RenderLayerCompositor::updateRootLayerPosition()
 {
-    if (m_rootContentLayer) {
-        m_rootContentLayer->setSize(m_renderView.frameView().contentsSize());
-        m_rootContentLayer->setPosition(m_renderView.frameView().positionForRootContentLayer());
-        m_rootContentLayer->setAnchorPoint(FloatPoint3D());
+    if (m_rootContentsLayer) {
+        m_rootContentsLayer->setSize(m_renderView.frameView().contentsSize());
+        m_rootContentsLayer->setPosition(m_renderView.frameView().positionForRootContentLayer());
+        m_rootContentsLayer->setAnchorPoint(FloatPoint3D());
     }
 
     updateScrollLayerClipping();
 
 #if ENABLE(RUBBER_BANDING)
-    if (m_contentShadowLayer && m_rootContentLayer) {
-        m_contentShadowLayer->setPosition(m_rootContentLayer->position());
-        m_contentShadowLayer->setSize(m_rootContentLayer->size());
+    if (m_contentShadowLayer && m_rootContentsLayer) {
+        m_contentShadowLayer->setPosition(m_rootContentsLayer->position());
+        m_contentShadowLayer->setSize(m_rootContentsLayer->size());
     }
 
     updateLayerForTopOverhangArea(m_layerForTopOverhangArea != nullptr);
@@ -2149,10 +2299,10 @@ static FullScreenDescendant isDescendantOfFullScreenLayer(const RenderLayer& lay
 {
     auto& document = layer.renderer().document();
 
-    if (!document.webkitIsFullScreen() || !document.fullScreenRenderer())
+    if (!document.fullscreenManager().isFullscreen() || !document.fullscreenManager().fullscreenRenderer())
         return FullScreenDescendant::NotApplicable;
 
-    auto* fullScreenLayer = document.fullScreenRenderer()->layer();
+    auto* fullScreenLayer = document.fullscreenManager().fullscreenRenderer()->layer();
     if (!fullScreenLayer) {
         ASSERT_NOT_REACHED();
         return FullScreenDescendant::NotApplicable;
@@ -2186,6 +2336,7 @@ bool RenderLayerCompositor::requiresOwnBackingStore(const RenderLayer& layer, co
         || requiresCompositingForPlugin(renderer, queryData)
         || requiresCompositingForEditableImage(renderer)
         || requiresCompositingForOverflowScrolling(layer, queryData)
+        || needsContentsCompositingLayer(layer)
         || renderer.isTransparent()
         || renderer.hasMask()
         || renderer.hasReflection()
@@ -2196,6 +2347,7 @@ bool RenderLayerCompositor::requiresOwnBackingStore(const RenderLayer& layer, co
     if (layer.mustCompositeForIndirectReasons()) {
         RenderLayer::IndirectCompositingReason reason = layer.indirectCompositingReason();
         return reason == RenderLayer::IndirectCompositingReason::Overlap
+            || reason == RenderLayer::IndirectCompositingReason::OverflowScrollPositioning
             || reason == RenderLayer::IndirectCompositingReason::Stacking
             || reason == RenderLayer::IndirectCompositingReason::BackgroundLayer
             || reason == RenderLayer::IndirectCompositingReason::GraphicalEffect
@@ -2252,7 +2404,7 @@ OptionSet<CompositingReason> RenderLayerCompositor::reasonsForCompositing(const
         reasons.add(renderer.isFixedPositioned() ? CompositingReason::PositionFixed : CompositingReason::PositionSticky);
 
     if (requiresCompositingForOverflowScrolling(*renderer.layer(), queryData))
-        reasons.add(CompositingReason::OverflowScrollingTouch);
+        reasons.add(CompositingReason::OverflowScrolling);
 
     switch (renderer.layer()->indirectCompositingReason()) {
     case RenderLayer::IndirectCompositingReason::None:
@@ -2260,6 +2412,9 @@ OptionSet<CompositingReason> RenderLayerCompositor::reasonsForCompositing(const
     case RenderLayer::IndirectCompositingReason::Stacking:
         reasons.add(CompositingReason::Stacking);
         break;
+    case RenderLayer::IndirectCompositingReason::OverflowScrollPositioning:
+        reasons.add(CompositingReason::OverflowScrollPositioning);
+        break;
     case RenderLayer::IndirectCompositingReason::Overlap:
         reasons.add(CompositingReason::Overlap);
         break;
@@ -2342,8 +2497,8 @@ const char* RenderLayerCompositor::logReasonsForCompositing(const RenderLayer& l
     if (reasons & CompositingReason::PositionSticky)
         return "position: sticky";
 
-    if (reasons & CompositingReason::OverflowScrollingTouch)
-        return "-webkit-overflow-scrolling: touch";
+    if (reasons & CompositingReason::OverflowScrolling)
+        return "async overflow scrolling";
 
     if (reasons & CompositingReason::Stacking)
         return "stacking";
@@ -2674,7 +2829,7 @@ bool RenderLayerCompositor::requiresCompositingForPosition(RenderLayerModelObjec
 #endif
 
     auto position = renderer.style().position();
-    bool isFixed = renderer.isOutOfFlowPositioned() && position == PositionType::Fixed;
+    bool isFixed = renderer.isFixedPositioned();
     if (isFixed && !layer.isStackingContext())
         return false;
     
@@ -2734,12 +2889,11 @@ bool RenderLayerCompositor::requiresCompositingForOverflowScrolling(const Render
 }
 
 // FIXME: why doesn't this handle the clipping cases?
-bool RenderLayerCompositor::requiresCompositingForIndirectReason(RenderLayerModelObject& renderer, bool hasCompositedDescendants, bool has3DTransformedDescendants, RenderLayer::IndirectCompositingReason& reason) const
+bool RenderLayerCompositor::requiresCompositingForIndirectReason(const RenderLayer& layer, const RenderLayer* compositingAncestor, bool hasCompositedDescendants, bool has3DTransformedDescendants, bool paintsIntoProvidedBacking, RenderLayer::IndirectCompositingReason& reason) const
 {
-    auto& layer = *downcast<RenderBoxModelObject>(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 descendants.
+    auto& renderer = layer.renderer();
     if (hasCompositedDescendants && (layer.isolatesCompositedBlending() || layer.transform() || renderer.createsGroup() || renderer.hasReflection())) {
         reason = RenderLayer::IndirectCompositingReason::GraphicalEffect;
         return true;
@@ -2759,6 +2913,13 @@ bool RenderLayerCompositor::requiresCompositingForIndirectReason(RenderLayerMode
         }
     }
 
+    if (!paintsIntoProvidedBacking && renderer.isAbsolutelyPositioned() && compositingAncestor && layer.hasCompositedScrollingAncestor()) {
+        if (layerContainingBlockCrossesCoordinatedScrollingBoundary(layer, *compositingAncestor)) {
+            reason = RenderLayer::IndirectCompositingReason::OverflowScrollPositioning;
+            return true;
+        }
+    }
+
     reason = RenderLayer::IndirectCompositingReason::None;
     return false;
 }
@@ -2787,15 +2948,12 @@ bool RenderLayerCompositor::isAsyncScrollableStickyLayer(const RenderLayer& laye
 
     auto* enclosingOverflowLayer = layer.enclosingOverflowClipLayer(ExcludeSelf);
 
-#if PLATFORM(IOS_FAMILY)
     if (enclosingOverflowLayer && enclosingOverflowLayer->hasCompositedScrollableOverflow()) {
         if (enclosingAcceleratedOverflowLayer)
             *enclosingAcceleratedOverflowLayer = enclosingOverflowLayer;
         return true;
     }
-#else
-    UNUSED_PARAM(enclosingAcceleratedOverflowLayer);
-#endif
+
     // If the layer is inside normal overflow, it's not async-scrollable.
     if (enclosingOverflowLayer)
         return false;
@@ -2817,7 +2975,7 @@ bool RenderLayerCompositor::isViewportConstrainedFixedOrStickyLayer(const Render
     if (layer.renderer().isStickilyPositioned())
         return isAsyncScrollableStickyLayer(layer);
 
-    if (layer.renderer().style().position() != PositionType::Fixed)
+    if (!layer.renderer().isFixedPositioned())
         return false;
 
     // FIXME: Handle fixed inside of a transform, which should not behave as fixed.
@@ -2831,7 +2989,7 @@ bool RenderLayerCompositor::isViewportConstrainedFixedOrStickyLayer(const Render
 
 bool RenderLayerCompositor::fixedLayerIntersectsViewport(const RenderLayer& layer) const
 {
-    ASSERT(layer.renderer().style().position() == PositionType::Fixed);
+    ASSERT(layer.renderer().isFixedPositioned());
 
     // Fixed position elements that are invisible in the current view don't get their own layer.
     // FIXME: We shouldn't have to check useFixedLayout() here; one of the viewport rects needs to give the correct answer.
@@ -2855,10 +3013,183 @@ bool RenderLayerCompositor::useCoordinatedScrollingForLayer(const RenderLayer& l
 
     if (auto* scrollingCoordinator = this->scrollingCoordinator())
         return scrollingCoordinator->coordinatesScrollingForOverflowLayer(layer);
-    
+
+    return false;
+}
+
+static RenderLayer* enclosingCompositedScrollingLayer(const RenderLayer& layer, const RenderLayer& intermediateLayer, bool& sawIntermediateLayer)
+{
+    const auto* currLayer = &layer;
+    while (currLayer) {
+        if (currLayer == &intermediateLayer)
+            sawIntermediateLayer = true;
+
+        if (currLayer->hasCompositedScrollableOverflow())
+            return const_cast<RenderLayer*>(currLayer);
+
+        currLayer = currLayer->parent();
+    }
+
+    return nullptr;
+}
+
+// Return true if overflowScrollLayer is in layer's containing block chain.
+static bool isScrolledByOverflowScrollLayer(const RenderLayer& layer, const RenderLayer& overflowScrollLayer)
+{
+    bool containingBlockCanSkipLayers = layer.renderer().isAbsolutelyPositioned();
+
+    for (const auto* currLayer = layer.parent(); currLayer; currLayer = currLayer->parent()) {
+        bool inContainingBlockChain = true;
+        if (containingBlockCanSkipLayers) {
+            inContainingBlockChain = currLayer->renderer().canContainAbsolutelyPositionedObjects();
+            if (inContainingBlockChain)
+                containingBlockCanSkipLayers = currLayer->renderer().isAbsolutelyPositioned();
+        }
+
+        if (currLayer == &overflowScrollLayer)
+            return inContainingBlockChain;
+    }
+
     return false;
 }
 
+static bool isNonScrolledLayerInsideScrolledCompositedAncestor(const RenderLayer& layer, const RenderLayer& compositedAncestor, const RenderLayer& scrollingAncestor)
+{
+    bool ancestorMovedByScroller = &compositedAncestor == &scrollingAncestor || isScrolledByOverflowScrollLayer(compositedAncestor, scrollingAncestor);
+    bool layerMovedByScroller = isScrolledByOverflowScrollLayer(layer, scrollingAncestor);
+
+    return ancestorMovedByScroller && !layerMovedByScroller;
+}
+
+bool RenderLayerCompositor::layerContainingBlockCrossesCoordinatedScrollingBoundary(const RenderLayer& layer, const RenderLayer& compositedAncestor)
+{
+    bool compositedAncestorIsInsideScroller = false;
+    auto* scrollingAncestor = enclosingCompositedScrollingLayer(layer, compositedAncestor, compositedAncestorIsInsideScroller);
+    if (!scrollingAncestor) {
+        ASSERT_NOT_REACHED(); // layer.hasCompositedScrollingAncestor() should guarantee we have one.
+        return false;
+    }
+    
+    if (!compositedAncestorIsInsideScroller)
+        return false;
+
+    return isNonScrolledLayerInsideScrolledCompositedAncestor(layer, compositedAncestor, *scrollingAncestor);
+}
+
+static void collectStationaryLayerRelatedOverflowNodes(const RenderLayer& layer, const RenderLayer& /*compositedAncestor*/, Vector<ScrollingNodeID>& scrollingNodes)
+{
+    ASSERT(layer.isComposited());
+    
+    auto appendOverflowLayerNodeID = [&scrollingNodes] (const RenderLayer& overflowLayer) {
+        ASSERT(overflowLayer.isComposited());
+        auto scrollingNodeID = overflowLayer.backing()->scrollingNodeIDForRole(ScrollCoordinationRole::Scrolling);
+        if (scrollingNodeID)
+            scrollingNodes.append(scrollingNodeID);
+        else
+            LOG(Scrolling, "Layer %p doesn't have scrolling node ID yet", &overflowLayer);
+    };
+
+    ASSERT(layer.renderer().isAbsolutelyPositioned());
+    bool containingBlockCanSkipLayers = true;
+
+    for (const auto* currLayer = layer.parent(); currLayer; currLayer = currLayer->parent()) {
+        bool inContainingBlockChain = true;
+        if (containingBlockCanSkipLayers) {
+            inContainingBlockChain = currLayer->renderer().canContainAbsolutelyPositionedObjects();
+            if (inContainingBlockChain)
+                containingBlockCanSkipLayers = currLayer->renderer().isAbsolutelyPositioned();
+        }
+
+        if (currLayer->hasCompositedScrollableOverflow()) {
+            appendOverflowLayerNodeID(*currLayer);
+            break;
+        }
+    }
+}
+
+
+ScrollPositioningBehavior RenderLayerCompositor::computeCoordinatedPositioningForLayer(const RenderLayer& layer) const
+{
+    if (layer.isRenderViewLayer())
+        return ScrollPositioningBehavior::None;
+
+    if (layer.renderer().isFixedPositioned())
+        return ScrollPositioningBehavior::None;
+    
+    if (!layer.hasCompositedScrollingAncestor())
+        return ScrollPositioningBehavior::None;
+
+    auto* scrollingCoordinator = this->scrollingCoordinator();
+    if (!scrollingCoordinator)
+        return ScrollPositioningBehavior::None;
+
+    auto* compositedAncestor = layer.ancestorCompositingLayer();
+    if (!compositedAncestor) {
+        ASSERT_NOT_REACHED();
+        return ScrollPositioningBehavior::None;
+    }
+
+    bool compositedAncestorIsInsideScroller = false;
+    auto* scrollingAncestor = enclosingCompositedScrollingLayer(layer, *compositedAncestor, compositedAncestorIsInsideScroller);
+    if (!scrollingAncestor) {
+        ASSERT_NOT_REACHED(); // layer.hasCompositedScrollingAncestor() should guarantee we have one.
+        return ScrollPositioningBehavior::None;
+    }
+
+    // There are two cases we have to deal with here:
+    // 1. There's a composited overflow:scroll in the parent chain between the renderer and its containing block, and the layer's
+    //    composited (z-order) ancestor is inside the scroller or is the scroller. In this case, we have to compensate for scroll position
+    //    changes to make the positioned layer stay in the same place. This only applies to position:absolute (since we handle fixed elsewhere).
+    if (layer.renderer().isAbsolutelyPositioned()) {
+        if (compositedAncestorIsInsideScroller && isNonScrolledLayerInsideScrolledCompositedAncestor(layer, *compositedAncestor, *scrollingAncestor))
+            return ScrollPositioningBehavior::Stationary;
+    }
+
+    // 2. The layer's containing block is the overflow or inside the overflow:scroll, but its z-order ancestor is
+    //    outside the overflow:scroll. In that case, we have to move the layer via the scrolling tree to make
+    //    it move along with the overflow scrolling.
+    if (!compositedAncestorIsInsideScroller && isScrolledByOverflowScrollLayer(layer, *scrollingAncestor))
+        return ScrollPositioningBehavior::Moves;
+
+    return ScrollPositioningBehavior::None;
+}
+
+static Vector<ScrollingNodeID> collectRelatedCoordinatedScrollingNodes(const RenderLayer& layer, ScrollPositioningBehavior positioningBehavior)
+{
+    Vector<ScrollingNodeID> overflowNodeData;
+
+    switch (positioningBehavior) {
+    case ScrollPositioningBehavior::Moves: {
+        // Collect all the composited scrollers between this layer and its composited ancestor.
+        auto* compositedAncestor = layer.ancestorCompositingLayer();
+        for (const auto* currLayer = layer.parent(); currLayer != compositedAncestor; currLayer = currLayer->parent()) {
+            if (currLayer->hasCompositedScrollableOverflow()) {
+                auto scrollingNodeID = currLayer->isComposited() ? currLayer->backing()->scrollingNodeIDForRole(ScrollCoordinationRole::Scrolling) : 0;
+                if (scrollingNodeID)
+                    overflowNodeData.append(scrollingNodeID);
+                else
+                    LOG(Scrolling, "Layer %p isn't composited or doesn't have scrolling node ID yet", &layer);
+            }
+        }
+        break;
+    }
+    case ScrollPositioningBehavior::Stationary: {
+        ASSERT(layer.renderer().isAbsolutelyPositioned());
+        // Collect all the composited scrollers between this layer and its composited ancestor.
+        auto* compositedAncestor = layer.ancestorCompositingLayer();
+        if (!compositedAncestor)
+            return overflowNodeData;
+        collectStationaryLayerRelatedOverflowNodes(layer, *compositedAncestor, overflowNodeData);
+        break;
+    }
+    case ScrollPositioningBehavior::None:
+        ASSERT_NOT_REACHED();
+        break;
+    }
+
+    return overflowNodeData;
+}
+
 bool RenderLayerCompositor::isLayerForIFrameWithScrollCoordinatedContents(const RenderLayer& layer) const
 {
     if (!is<RenderWidget>(layer.renderer()))
@@ -3050,7 +3381,7 @@ bool RenderLayerCompositor::shouldCompositeOverflowControls() const
     if (m_overflowControlsHostLayer && isMainFrameCompositor())
         return true;
 
-#if !USE(COORDINATED_GRAPHICS_THREADED)
+#if !USE(COORDINATED_GRAPHICS)
     if (!frameView.hasOverlayScrollbars())
         return false;
 #endif
@@ -3120,7 +3451,7 @@ GraphicsLayer* RenderLayerCompositor::updateLayerForTopOverhangArea(bool wantsLa
     if (!m_layerForTopOverhangArea) {
         m_layerForTopOverhangArea = GraphicsLayer::create(graphicsLayerFactory(), *this);
         m_layerForTopOverhangArea->setName("top overhang");
-        m_scrollLayer->addChildBelow(*m_layerForTopOverhangArea, m_rootContentLayer.get());
+        m_scrolledContentsLayer->addChildBelow(*m_layerForTopOverhangArea, m_rootContentsLayer.get());
     }
 
     return m_layerForTopOverhangArea.get();
@@ -3139,10 +3470,10 @@ GraphicsLayer* RenderLayerCompositor::updateLayerForBottomOverhangArea(bool want
     if (!m_layerForBottomOverhangArea) {
         m_layerForBottomOverhangArea = GraphicsLayer::create(graphicsLayerFactory(), *this);
         m_layerForBottomOverhangArea->setName("bottom overhang");
-        m_scrollLayer->addChildBelow(*m_layerForBottomOverhangArea, m_rootContentLayer.get());
+        m_scrolledContentsLayer->addChildBelow(*m_layerForBottomOverhangArea, m_rootContentsLayer.get());
     }
 
-    m_layerForBottomOverhangArea->setPosition(FloatPoint(0, m_rootContentLayer->size().height() + m_renderView.frameView().headerHeight()
+    m_layerForBottomOverhangArea->setPosition(FloatPoint(0, m_rootContentsLayer->size().height() + m_renderView.frameView().headerHeight()
         + m_renderView.frameView().footerHeight() + m_renderView.frameView().topContentInset()));
     return m_layerForBottomOverhangArea.get();
 }
@@ -3167,7 +3498,7 @@ GraphicsLayer* RenderLayerCompositor::updateLayerForHeader(bool wantsLayer)
     if (!m_layerForHeader) {
         m_layerForHeader = GraphicsLayer::create(graphicsLayerFactory(), *this);
         m_layerForHeader->setName("header");
-        m_scrollLayer->addChildAbove(*m_layerForHeader, m_rootContentLayer.get());
+        m_scrolledContentsLayer->addChildAbove(*m_layerForHeader, m_rootContentsLayer.get());
         m_renderView.frameView().addPaintPendingMilestones(DidFirstFlushForHeaderLayer);
     }
 
@@ -3204,10 +3535,10 @@ GraphicsLayer* RenderLayerCompositor::updateLayerForFooter(bool wantsLayer)
     if (!m_layerForFooter) {
         m_layerForFooter = GraphicsLayer::create(graphicsLayerFactory(), *this);
         m_layerForFooter->setName("footer");
-        m_scrollLayer->addChildAbove(*m_layerForFooter, m_rootContentLayer.get());
+        m_scrolledContentsLayer->addChildAbove(*m_layerForFooter, m_rootContentsLayer.get());
     }
 
-    float totalContentHeight = m_rootContentLayer->size().height() + m_renderView.frameView().headerHeight() + m_renderView.frameView().footerHeight();
+    float totalContentHeight = m_rootContentsLayer->size().height() + m_renderView.frameView().headerHeight() + m_renderView.frameView().footerHeight();
     m_layerForFooter->setPosition(FloatPoint(0, FrameView::yPositionForFooterLayer(m_renderView.frameView().scrollPosition(),
         m_renderView.frameView().topContentInset(), totalContentHeight, m_renderView.frameView().footerHeight())));
     m_layerForFooter->setAnchorPoint(FloatPoint3D());
@@ -3333,12 +3664,12 @@ void RenderLayerCompositor::updateOverflowControlsLayers()
         if (!m_contentShadowLayer) {
             m_contentShadowLayer = GraphicsLayer::create(graphicsLayerFactory(), *this);
             m_contentShadowLayer->setName("content shadow");
-            m_contentShadowLayer->setSize(m_rootContentLayer->size());
-            m_contentShadowLayer->setPosition(m_rootContentLayer->position());
+            m_contentShadowLayer->setSize(m_rootContentsLayer->size());
+            m_contentShadowLayer->setPosition(m_rootContentsLayer->position());
             m_contentShadowLayer->setAnchorPoint(FloatPoint3D());
             m_contentShadowLayer->setCustomAppearance(GraphicsLayer::CustomAppearance::ScrollingShadow);
 
-            m_scrollLayer->addChildBelow(*m_contentShadowLayer, m_rootContentLayer.get());
+            m_scrolledContentsLayer->addChildBelow(*m_contentShadowLayer, m_rootContentsLayer.get());
         }
     } else
         GraphicsLayer::unparentAndClear(m_contentShadowLayer);
@@ -3409,18 +3740,18 @@ void RenderLayerCompositor::ensureRootLayer()
     if (expectedAttachment == m_rootLayerAttachment)
          return;
 
-    if (!m_rootContentLayer) {
-        m_rootContentLayer = GraphicsLayer::create(graphicsLayerFactory(), *this);
-        m_rootContentLayer->setName("content root");
+    if (!m_rootContentsLayer) {
+        m_rootContentsLayer = GraphicsLayer::create(graphicsLayerFactory(), *this);
+        m_rootContentsLayer->setName("content root");
         IntRect overflowRect = snappedIntRect(m_renderView.layoutOverflowRect());
-        m_rootContentLayer->setSize(FloatSize(overflowRect.maxX(), overflowRect.maxY()));
-        m_rootContentLayer->setPosition(FloatPoint());
+        m_rootContentsLayer->setSize(FloatSize(overflowRect.maxX(), overflowRect.maxY()));
+        m_rootContentsLayer->setPosition(FloatPoint());
 
 #if PLATFORM(IOS_FAMILY)
         // Page scale is applied above this on iOS, so we'll just say that our root layer applies it.
         auto& frame = m_renderView.frameView().frame();
         if (frame.isMainFrame())
-            m_rootContentLayer->setAppliesPageScale();
+            m_rootContentsLayer->setAppliesPageScale();
 #endif
 
         // Need to clip to prevent transformed content showing outside this frame
@@ -3429,35 +3760,40 @@ void RenderLayerCompositor::ensureRootLayer()
 
     if (requiresScrollLayer(expectedAttachment)) {
         if (!m_overflowControlsHostLayer) {
-            ASSERT(!m_scrollLayer);
+            ASSERT(!m_scrolledContentsLayer);
             ASSERT(!m_clipLayer);
 
             // Create a layer to host the clipping layer and the overflow controls layers.
             m_overflowControlsHostLayer = GraphicsLayer::create(graphicsLayerFactory(), *this);
             m_overflowControlsHostLayer->setName("overflow controls host");
 
-            auto scrollLayerType = GraphicsLayer::Type::Normal;
+            m_scrolledContentsLayer = GraphicsLayer::create(graphicsLayerFactory(), *this);
+            m_scrolledContentsLayer->setName("scrolled contents");
+            m_scrolledContentsLayer->setAnchorPoint({ });
+
 #if PLATFORM(IOS_FAMILY)
-            if (m_renderView.settings().asyncFrameScrollingEnabled())
-                scrollLayerType = GraphicsLayer::Type::Scrolling;
-#endif
-            m_scrollLayer = GraphicsLayer::create(graphicsLayerFactory(), *this, scrollLayerType);
-            m_scrollLayer->setName("frame scrolling");
+            if (m_renderView.settings().asyncFrameScrollingEnabled()) {
+                m_scrollContainerLayer = GraphicsLayer::create(graphicsLayerFactory(), *this, GraphicsLayer::Type::ScrollContainer);
 
-            if (scrollLayerType == GraphicsLayer::Type::Scrolling) {
-                // Scroll layer clips so there is no need for a separate clipping layer.
-                m_overflowControlsHostLayer->addChild(*m_scrollLayer);
-            } else {
+                m_scrollContainerLayer->setName("scroll container");
+                m_scrollContainerLayer->setMasksToBounds(true);
+                m_scrollContainerLayer->setAnchorPoint({ });
+
+                m_scrollContainerLayer->addChild(*m_scrolledContentsLayer);
+                m_overflowControlsHostLayer->addChild(*m_scrollContainerLayer);
+            }
+#endif
+            if (!m_scrollContainerLayer) {
                 m_clipLayer = GraphicsLayer::create(graphicsLayerFactory(), *this);
                 m_clipLayer->setName("frame clipping");
                 m_clipLayer->setMasksToBounds(true);
-                m_clipLayer->setAnchorPoint(FloatPoint3D());
+                m_clipLayer->setAnchorPoint({ });
 
-                m_clipLayer->addChild(*m_scrollLayer);
+                m_clipLayer->addChild(*m_scrolledContentsLayer);
                 m_overflowControlsHostLayer->addChild(*m_clipLayer);
             }
 
-            m_scrollLayer->addChild(*m_rootContentLayer);
+            m_scrolledContentsLayer->addChild(*m_rootContentsLayer);
 
             updateScrollLayerClipping();
             updateOverflowControlsLayers();
@@ -3471,7 +3807,8 @@ void RenderLayerCompositor::ensureRootLayer()
         if (m_overflowControlsHostLayer) {
             GraphicsLayer::unparentAndClear(m_overflowControlsHostLayer);
             GraphicsLayer::unparentAndClear(m_clipLayer);
-            GraphicsLayer::unparentAndClear(m_scrollLayer);
+            GraphicsLayer::unparentAndClear(m_scrollContainerLayer);
+            GraphicsLayer::unparentAndClear(m_scrolledContentsLayer);
         }
     }
 
@@ -3484,7 +3821,7 @@ void RenderLayerCompositor::ensureRootLayer()
 
 void RenderLayerCompositor::destroyRootLayer()
 {
-    if (!m_rootContentLayer)
+    if (!m_rootContentsLayer)
         return;
 
     detachRootLayer();
@@ -3517,17 +3854,18 @@ void RenderLayerCompositor::destroyRootLayer()
     if (m_overflowControlsHostLayer) {
         GraphicsLayer::unparentAndClear(m_overflowControlsHostLayer);
         GraphicsLayer::unparentAndClear(m_clipLayer);
-        GraphicsLayer::unparentAndClear(m_scrollLayer);
+        GraphicsLayer::unparentAndClear(m_scrollContainerLayer);
+        GraphicsLayer::unparentAndClear(m_scrolledContentsLayer);
     }
-    ASSERT(!m_scrollLayer);
-    GraphicsLayer::unparentAndClear(m_rootContentLayer);
+    ASSERT(!m_scrolledContentsLayer);
+    GraphicsLayer::unparentAndClear(m_rootContentsLayer);
 
     m_layerUpdater = nullptr;
 }
 
 void RenderLayerCompositor::attachRootLayer(RootLayerAttachment attachment)
 {
-    if (!m_rootContentLayer)
+    if (!m_rootContentsLayer)
         return;
 
     LOG(Compositing, "RenderLayerCompositor %p attachRootLayer %d", this, attachment);
@@ -3539,8 +3877,6 @@ void RenderLayerCompositor::attachRootLayer(RootLayerAttachment attachment)
         case RootLayerAttachedViaChromeClient: {
             auto& frame = m_renderView.frameView().frame();
             page().chrome().client().attachRootGraphicsLayer(frame, rootGraphicsLayer());
-            if (frame.isMainFrame())
-                page().chrome().client().attachViewOverlayGraphicsLayer(frame, &page().pageOverlayController().layerWithViewOverlays());
             break;
         }
         case RootLayerAttachedViaEnclosingFrame: {
@@ -3556,14 +3892,14 @@ void RenderLayerCompositor::attachRootLayer(RootLayerAttachment attachment)
     rootLayerAttachmentChanged();
     
     if (m_shouldFlushOnReattach) {
-        scheduleLayerFlushNow();
+        scheduleLayerFlush();
         m_shouldFlushOnReattach = false;
     }
 }
 
 void RenderLayerCompositor::detachRootLayer()
 {
-    if (!m_rootContentLayer || m_rootLayerAttachment == RootLayerUnattached)
+    if (!m_rootContentsLayer || m_rootLayerAttachment == RootLayerUnattached)
         return;
 
     switch (m_rootLayerAttachment) {
@@ -3573,12 +3909,12 @@ void RenderLayerCompositor::detachRootLayer()
         if (m_overflowControlsHostLayer)
             m_overflowControlsHostLayer->removeFromParent();
         else
-            m_rootContentLayer->removeFromParent();
+            m_rootContentsLayer->removeFromParent();
 
         if (auto* ownerElement = m_renderView.document().ownerElement())
             ownerElement->scheduleInvalidateStyleAndLayerComposition();
 
-        if (auto frameRootScrollingNodeID = m_renderView.frameView().scrollLayerID()) {
+        if (auto frameRootScrollingNodeID = m_renderView.frameView().scrollingNodeID()) {
             if (auto* scrollingCoordinator = this->scrollingCoordinator())
                 scrollingCoordinator->unparentNode(frameRootScrollingNodeID);
         }
@@ -3587,10 +3923,6 @@ void RenderLayerCompositor::detachRootLayer()
     case RootLayerAttachedViaChromeClient: {
         auto& frame = m_renderView.frameView().frame();
         page().chrome().client().attachRootGraphicsLayer(frame, nullptr);
-        if (frame.isMainFrame()) {
-            page().chrome().client().attachViewOverlayGraphicsLayer(frame, nullptr);
-            page().pageOverlayController().willDetachRootLayer();
-        }
     }
     break;
     case RootLayerUnattached:
@@ -3626,7 +3958,7 @@ void RenderLayerCompositor::rootLayerAttachmentChanged()
         return;
 
     Ref<GraphicsLayer> overlayHost = page().pageOverlayController().layerWithDocumentOverlays();
-    m_rootContentLayer->addChild(WTFMove(overlayHost));
+    m_rootContentsLayer->addChild(WTFMove(overlayHost));
 }
 
 void RenderLayerCompositor::notifyIFramesOfCompositingChange()
@@ -3676,49 +4008,14 @@ void RenderLayerCompositor::deviceOrPageScaleFactorChanged()
         rootLayer->noteDeviceOrPageScaleFactorChangedIncludingDescendants();
 }
 
-static bool canCoordinateScrollingForLayer(const RenderLayer& layer)
-{
-    return (layer.isRenderViewLayer() || layer.parent()) && layer.isComposited();
-}
-
-void RenderLayerCompositor::updateScrollCoordinatedStatus(RenderLayer& layer, OptionSet<ScrollingNodeChangeFlags> changes)
-{
-    OptionSet<ScrollCoordinationRole> coordinationRoles;
-    if (isViewportConstrainedFixedOrStickyLayer(layer))
-        coordinationRoles.add(ScrollCoordinationRole::ViewportConstrained);
-
-    if (useCoordinatedScrollingForLayer(layer))
-        coordinationRoles.add(ScrollCoordinationRole::Scrolling);
-
-    if (isLayerForIFrameWithScrollCoordinatedContents(layer)) {
-        // We never expect a RenderIFrame to have scrollable overflow.
-        ASSERT(!coordinationRoles.contains(ScrollCoordinationRole::Scrolling));
-        coordinationRoles.add(ScrollCoordinationRole::FrameHosting);
-    }
-
-    if (layer.isComposited())
-        layer.backing()->setIsScrollCoordinatedWithViewportConstrainedRole(coordinationRoles.contains(ScrollCoordinationRole::ViewportConstrained));
-
-    if (coordinationRoles && canCoordinateScrollingForLayer(layer)) {
-        if (m_scrollCoordinatedLayers.add(&layer).isNewEntry)
-            m_subframeScrollLayersNeedReattach = true;
-
-        updateScrollCoordinatedLayer(layer, coordinationRoles, changes);
-    } else
-        removeFromScrollCoordinatedLayers(layer);
-}
-
 void RenderLayerCompositor::removeFromScrollCoordinatedLayers(RenderLayer& layer)
 {
-    if (!m_scrollCoordinatedLayers.contains(&layer))
-        return;
-
-    m_subframeScrollLayersNeedReattach = true;
-    
-    m_scrollCoordinatedLayers.remove(&layer);
-    m_scrollCoordinatedLayersNeedingUpdate.remove(&layer);
+#if PLATFORM(IOS_FAMILY)
+    if (m_legacyScrollingLayerCoordinator)
+        m_legacyScrollingLayerCoordinator->removeLayer(layer);
+#endif
 
-    detachScrollCoordinatedLayer(layer, { ScrollCoordinationRole::Scrolling, ScrollCoordinationRole::ViewportConstrained, ScrollCoordinationRole::FrameHosting });
+    detachScrollCoordinatedLayer(layer, { ScrollCoordinationRole::Scrolling, ScrollCoordinationRole::ViewportConstrained, ScrollCoordinationRole::FrameHosting, ScrollCoordinationRole::Positioning });
 }
 
 FixedPositionViewportConstraints RenderLayerCompositor::computeFixedViewportConstraints(RenderLayer& layer) const
@@ -3759,11 +4056,6 @@ FixedPositionViewportConstraints RenderLayerCompositor::computeFixedViewportCons
 StickyPositionViewportConstraints RenderLayerCompositor::computeStickyViewportConstraints(RenderLayer& layer) const
 {
     ASSERT(layer.isComposited());
-#if !PLATFORM(IOS_FAMILY)
-    // We should never get here for stickies constrained by an enclosing clipping layer.
-    // FIXME: Why does this assertion fail on iOS?
-    ASSERT(!layer.enclosingOverflowClipLayer(ExcludeSelf));
-#endif
 
     auto& renderer = downcast<RenderBoxModelObject>(layer.renderer());
 
@@ -3778,72 +4070,6 @@ StickyPositionViewportConstraints RenderLayerCompositor::computeStickyViewportCo
     return constraints;
 }
 
-static ScrollingNodeID enclosingScrollingNodeID(RenderLayer& layer, IncludeSelfOrNot includeSelf)
-{
-    auto* currLayer = includeSelf == IncludeSelf ? &layer : layer.parent();
-    while (currLayer) {
-        if (auto* backing = currLayer->backing()) {
-            if (ScrollingNodeID nodeID = backing->scrollingNodeIDForChildren())
-                return nodeID;
-        }
-        currLayer = currLayer->parent();
-    }
-
-    return 0;
-}
-
-static Optional<ScrollingNodeID> scrollCoordinatedAncestorInParentOfFrame(Frame& frame)
-{
-    if (!frame.document() || !frame.view())
-        return { };
-
-    // Find the frame's enclosing layer in our render tree.
-    auto* ownerElement = frame.document()->ownerElement();
-    auto* frameRenderer = ownerElement ? ownerElement->renderer() : nullptr;
-    if (!frameRenderer || !is<RenderWidget>(frameRenderer))
-        return { };
-
-    auto& widgetRenderer = downcast<RenderWidget>(*frameRenderer);
-    if (!widgetRenderer.hasLayer() || !widgetRenderer.layer()->isComposited()) {
-        LOG(Scrolling, "scrollCoordinatedAncestorInParentOfFrame: frame renderer has no layer or is not composited.");
-        return { };
-    }
-
-    if (auto frameHostingNodeID = widgetRenderer.layer()->backing()->scrollingNodeIDForRole(ScrollCoordinationRole::FrameHosting))
-        return frameHostingNodeID;
-
-    return { };
-}
-
-void RenderLayerCompositor::reattachSubframeScrollLayers()
-{
-    if (!m_subframeScrollLayersNeedReattach)
-        return;
-    
-    m_subframeScrollLayersNeedReattach = false;
-
-    auto* scrollingCoordinator = this->scrollingCoordinator();
-
-    for (Frame* child = m_renderView.frameView().frame().tree().firstChild(); child; child = child->tree().nextSibling()) {
-        if (!child->document() || !child->view())
-            continue;
-
-        // Ignore frames that are not scroll-coordinated.
-        auto* childFrameView = child->view();
-        ScrollingNodeID frameScrollingNodeID = childFrameView->scrollLayerID();
-        if (!frameScrollingNodeID)
-            continue;
-
-        auto parentNodeID = scrollCoordinatedAncestorInParentOfFrame(*child);
-        if (!parentNodeID) {
-            LOG(Scrolling, "RenderLayerCompositor %p reattachSubframeScrollLayers: failed to find scrolling node parent for frame with nodeID %" PRIu64 ".", this, frameScrollingNodeID);
-            continue;
-        }
-
-        scrollingCoordinator->insertNode(child->isMainFrame() ? ScrollingNodeType::MainFrame : ScrollingNodeType::Subframe, frameScrollingNodeID, parentNodeID.value());
-    }
-}
-
 static inline ScrollCoordinationRole scrollCoordinationRoleForNodeType(ScrollingNodeType nodeType)
 {
     switch (nodeType) {
@@ -3856,12 +4082,14 @@ static inline ScrollCoordinationRole scrollCoordinationRoleForNodeType(Scrolling
     case ScrollingNodeType::Fixed:
     case ScrollingNodeType::Sticky:
         return ScrollCoordinationRole::ViewportConstrained;
+    case ScrollingNodeType::Positioned:
+        return ScrollCoordinationRole::Positioning;
     }
     ASSERT_NOT_REACHED();
     return ScrollCoordinationRole::Scrolling;
 }
 
-ScrollingNodeID RenderLayerCompositor::attachScrollingNode(RenderLayer& layer, ScrollingNodeType nodeType, Optional<ScrollingNodeID> parentNodeID)
+ScrollingNodeID RenderLayerCompositor::attachScrollingNode(RenderLayer& layer, ScrollingNodeType nodeType, ScrollingTreeState& treeState)
 {
     auto* scrollingCoordinator = this->scrollingCoordinator();
     auto* backing = layer.backing();
@@ -3870,24 +4098,36 @@ ScrollingNodeID RenderLayerCompositor::attachScrollingNode(RenderLayer& layer, S
     if (!backing)
         return 0;
 
+    ASSERT(treeState.parentNodeID || nodeType == ScrollingNodeType::Subframe);
+    ASSERT_IMPLIES(nodeType == ScrollingNodeType::MainFrame, !treeState.parentNodeID.value());
+
     ScrollCoordinationRole role = scrollCoordinationRoleForNodeType(nodeType);
     ScrollingNodeID nodeID = backing->scrollingNodeIDForRole(role);
     if (!nodeID)
         nodeID = scrollingCoordinator->uniqueScrollingNodeID();
 
-    LOG_WITH_STREAM(Scrolling, stream << "RenderLayerCompositor " << this << " attachScrollingNode " << nodeID << " type " << nodeType << " parent " << parentNodeID.valueOr(0));
+    LOG_WITH_STREAM(Scrolling, stream << "RenderLayerCompositor " << this << " attachScrollingNode " << nodeID << " (layer " << backing->graphicsLayer()->primaryLayerID() << ") type " << nodeType << " parent " << treeState.parentNodeID.valueOr(0));
 
-    if (nodeType == ScrollingNodeType::Subframe && !parentNodeID)
+    if (nodeType == ScrollingNodeType::Subframe && !treeState.parentNodeID)
         nodeID = scrollingCoordinator->createNode(nodeType, nodeID);
-    else
-        nodeID = scrollingCoordinator->insertNode(nodeType, nodeID, parentNodeID.valueOr(0));
+    else {
+        auto newNodeID = scrollingCoordinator->insertNode(nodeType, nodeID, treeState.parentNodeID.valueOr(0), treeState.nextChildIndex);
+        if (newNodeID != nodeID) {
+            // We'll get a new nodeID if the type changed (and not if the node is new).
+            scrollingCoordinator->unparentChildrenAndDestroyNode(nodeID);
+            m_scrollingNodeToLayerMap.remove(nodeID);
+        }
+        nodeID = newNodeID;
+    }
 
+    ASSERT(nodeID);
     if (!nodeID)
         return 0;
-
+    
     backing->setScrollingNodeIDForRole(nodeID, role);
     m_scrollingNodeToLayerMap.add(nodeID, &layer);
-
+    
+    ++treeState.nextChildIndex;
     return nodeID;
 }
 
@@ -3924,151 +4164,242 @@ void RenderLayerCompositor::detachScrollCoordinatedLayer(RenderLayer& layer, Opt
     if (roles.contains(ScrollCoordinationRole::ViewportConstrained))
         detachScrollCoordinatedLayerWithRole(layer, *scrollingCoordinator, ScrollCoordinationRole::ViewportConstrained);
 
+    if (roles.contains(ScrollCoordinationRole::Positioning))
+        detachScrollCoordinatedLayerWithRole(layer, *scrollingCoordinator, ScrollCoordinationRole::Positioning);
+
     backing->detachFromScrollingCoordinator(roles);
 }
 
-void RenderLayerCompositor::updateScrollCoordinationForThisFrame(Optional<ScrollingNodeID> parentNodeID, OptionSet<ScrollingNodeChangeFlags> changes)
+ScrollingNodeID RenderLayerCompositor::updateScrollCoordinationForLayer(RenderLayer& layer, ScrollingTreeState& treeState, OptionSet<ScrollCoordinationRole> roles, OptionSet<ScrollingNodeChangeFlags> changes)
+{
+    bool isViewportConstrained = roles.contains(ScrollCoordinationRole::ViewportConstrained);
+#if PLATFORM(IOS_FAMILY)
+    if (m_legacyScrollingLayerCoordinator) {
+        if (isViewportConstrained)
+            m_legacyScrollingLayerCoordinator->addViewportConstrainedLayer(layer);
+        else
+            m_legacyScrollingLayerCoordinator->removeViewportConstrainedLayer(layer);
+    }
+#endif
+
+    // GraphicsLayers need to know whether they are viewport-constrained.
+    layer.backing()->setIsScrollCoordinatedWithViewportConstrainedRole(isViewportConstrained);
+
+    if (!hasCoordinatedScrolling()) {
+        // If this frame isn't coordinated, it cannot contain any scrolling tree nodes.
+        return 0;
+    }
+
+    auto newNodeID = treeState.parentNodeID.valueOr(0);
+
+    ScrollingTreeState childTreeState;
+    ScrollingTreeState* currentTreeState = &treeState;
+
+    // If is fixed or sticky, it's the parent scrolling node for scrolling/frame hosting.
+    if (roles.contains(ScrollCoordinationRole::ViewportConstrained)) {
+        newNodeID = updateScrollingNodeForViewportConstrainedRole(layer, *currentTreeState, changes);
+        // ViewportConstrained nodes are the parent of same-layer scrolling nodes, so adjust the ScrollingTreeState.
+        childTreeState.parentNodeID = newNodeID;
+        currentTreeState = &childTreeState;
+    } else
+        detachScrollCoordinatedLayer(layer, ScrollCoordinationRole::ViewportConstrained);
+
+    // If there's a positioning node, it's the parent scrolling node for scrolling/frame hosting.
+    if (roles.contains(ScrollCoordinationRole::Positioning)) {
+        newNodeID = updateScrollingNodeForPositioningRole(layer, *currentTreeState, changes);
+        childTreeState.parentNodeID = newNodeID;
+        currentTreeState = &childTreeState;
+    } else
+        detachScrollCoordinatedLayer(layer, ScrollCoordinationRole::Positioning);
+
+    if (roles.contains(ScrollCoordinationRole::Scrolling))
+        newNodeID = updateScrollingNodeForScrollingRole(layer, *currentTreeState, changes);
+    else
+        detachScrollCoordinatedLayer(layer, ScrollCoordinationRole::Scrolling);
+
+    if (roles.contains(ScrollCoordinationRole::FrameHosting))
+        newNodeID = updateScrollingNodeForFrameHostingRole(layer, *currentTreeState, changes);
+    else
+        detachScrollCoordinatedLayer(layer, ScrollCoordinationRole::FrameHosting);
+
+    return newNodeID;
+}
+
+ScrollingNodeID RenderLayerCompositor::updateScrollingNodeForViewportConstrainedRole(RenderLayer& layer, ScrollingTreeState& treeState, OptionSet<ScrollingNodeChangeFlags> changes)
 {
     auto* scrollingCoordinator = this->scrollingCoordinator();
-    FrameView& frameView = m_renderView.frameView();
-    ASSERT(scrollingCoordinator->coordinatesScrollingForFrameView(frameView));
 
-    ScrollingNodeID nodeID = attachScrollingNode(*m_renderView.layer(), m_renderView.frame().isMainFrame() ? ScrollingNodeType::MainFrame : ScrollingNodeType::Subframe, parentNodeID);
+    auto nodeType = ScrollingNodeType::Fixed;
+    if (layer.renderer().style().position() == PositionType::Sticky)
+        nodeType = ScrollingNodeType::Sticky;
+    else
+        ASSERT(layer.renderer().isFixedPositioned());
+
+    auto newNodeID = attachScrollingNode(layer, nodeType, treeState);
+    if (!newNodeID) {
+        ASSERT_NOT_REACHED();
+        return treeState.parentNodeID.valueOr(0);
+    }
+
+    LOG_WITH_STREAM(Compositing, stream << "Registering ViewportConstrained " << nodeType << " node " << newNodeID << " (layer " << layer.backing()->graphicsLayer()->primaryLayerID() << ") as child of " << treeState.parentNodeID.valueOr(0));
 
     if (changes & ScrollingNodeChangeFlags::Layer)
-        scrollingCoordinator->setNodeLayers(nodeID, m_scrollLayer.get(), m_rootContentLayer.get(), fixedRootBackgroundLayer(), clipLayer());
+        scrollingCoordinator->setNodeLayers(newNodeID, { layer.backing()->graphicsLayer() });
 
     if (changes & ScrollingNodeChangeFlags::LayerGeometry) {
-        ScrollingCoordinator::ScrollingGeometry scrollingGeometry;
-        // FIXME(https://webkit.org/b/172917): Pass parentRelativeScrollableRect?
-        scrollingGeometry.scrollOrigin = frameView.scrollOrigin();
-        scrollingGeometry.scrollableAreaSize = frameView.visibleContentRect().size();
-        scrollingGeometry.contentSize = frameView.totalContentsSize();
-        scrollingGeometry.reachableContentSize = frameView.totalContentsSize();
-#if ENABLE(CSS_SCROLL_SNAP)
-        frameView.updateSnapOffsets();
-        scrollingCoordinator->updateScrollSnapPropertiesWithFrameView(frameView);
-#endif
-        scrollingCoordinator->setScrollingNodeGeometry(nodeID, scrollingGeometry);
+        switch (nodeType) {
+        case ScrollingNodeType::Fixed:
+            scrollingCoordinator->setViewportConstraintedNodeConstraints(newNodeID, computeFixedViewportConstraints(layer));
+            break;
+        case ScrollingNodeType::Sticky:
+            scrollingCoordinator->setViewportConstraintedNodeConstraints(newNodeID, computeStickyViewportConstraints(layer));
+            break;
+        default:
+            break;
+        }
     }
+
+    return newNodeID;
 }
 
-void RenderLayerCompositor::updateScrollCoordinatedLayer(RenderLayer& layer, OptionSet<ScrollCoordinationRole> roles, OptionSet<ScrollingNodeChangeFlags> changes)
+LayoutRect RenderLayerCompositor::rootParentRelativeScrollableRect() const
 {
-    bool isRenderViewLayer = layer.isRenderViewLayer();
+    auto& frameView = m_renderView.frameView();
 
-    ASSERT(m_scrollCoordinatedLayers.contains(&layer));
-    ASSERT(layer.isComposited());
+    if (m_renderView.frame().isMainFrame())
+        return frameView.frameRect();
+    
+    return LayoutRect({ }, LayoutSize(frameView.size()));
+}
 
-    auto* scrollingCoordinator = this->scrollingCoordinator();
-    if (!scrollingCoordinator || !scrollingCoordinator->coordinatesScrollingForFrameView(m_renderView.frameView()))
-        return;
+LayoutRect RenderLayerCompositor::parentRelativeScrollableRect(const RenderLayer& layer, const RenderLayer* ancestorLayer) const
+{
+    // FIXME: ancestorLayer needs to be always non-null, so should become a reference.
+    if (!ancestorLayer)
+        return LayoutRect({ }, LayoutSize(layer.visibleSize()));
 
-    if (!m_renderView.frame().isMainFrame()) {
-        auto parentDocumentHostingNodeID = scrollCoordinatedAncestorInParentOfFrame(m_renderView.frame());
-        updateScrollCoordinationForThisFrame(parentDocumentHostingNodeID, changes);
-        if (!(roles.contains(ScrollCoordinationRole::ViewportConstrained)) && isRenderViewLayer)
-            return;
+    LayoutRect scrollableRect;
+    if (is<RenderBox>(layer.renderer()))
+        scrollableRect = downcast<RenderBox>(layer.renderer()).paddingBoxRect();
+
+    auto offset = layer.convertToLayerCoords(ancestorLayer, scrollableRect.location()); // FIXME: broken for columns.
+    scrollableRect.setLocation(offset);
+    return scrollableRect;
+}
+
+void RenderLayerCompositor::updateScrollingNodeLayers(ScrollingNodeID nodeID, RenderLayer& layer, ScrollingCoordinator& scrollingCoordinator)
+{
+    if (layer.isRenderViewLayer()) {
+        FrameView& frameView = m_renderView.frameView();
+        scrollingCoordinator.setNodeLayers(nodeID, { nullptr,
+            scrollContainerLayer(), scrolledContentsLayer(),
+            fixedRootBackgroundLayer(), clipLayer(), rootContentsLayer(),
+            frameView.layerForHorizontalScrollbar(), frameView.layerForVerticalScrollbar() });
+    } else {
+        auto& backing = *layer.backing();
+        scrollingCoordinator.setNodeLayers(nodeID, { backing.graphicsLayer(),
+            backing.scrollContainerLayer(), backing.scrolledContentsLayer(),
+            nullptr, nullptr, nullptr,
+            layer.layerForHorizontalScrollbar(), layer.layerForVerticalScrollbar() });
     }
+}
 
-    ScrollingNodeID parentNodeID = enclosingScrollingNodeID(layer, ExcludeSelf);
-    if (!parentNodeID && !isRenderViewLayer)
-        return;
+ScrollingNodeID RenderLayerCompositor::updateScrollingNodeForScrollingRole(RenderLayer& layer, ScrollingTreeState& treeState, OptionSet<ScrollingNodeChangeFlags> changes)
+{
+    auto* scrollingCoordinator = this->scrollingCoordinator();
 
-    auto* backing = layer.backing();
+    ScrollingNodeID newNodeID = 0;
 
-    // Always call this even if the backing is already attached because the parent may have changed.
-    // If a node plays both roles, fixed/sticky is always the ancestor node of scrolling.
-    if (roles.contains(ScrollCoordinationRole::ViewportConstrained)) {
-        ScrollingNodeType nodeType = ScrollingNodeType::MainFrame;
-        if (layer.renderer().isFixedPositioned())
-            nodeType = ScrollingNodeType::Fixed;
-        else if (layer.renderer().style().position() == PositionType::Sticky)
-            nodeType = ScrollingNodeType::Sticky;
-        else
-            ASSERT_NOT_REACHED();
+    if (layer.isRenderViewLayer()) {
+        FrameView& frameView = m_renderView.frameView();
+        ASSERT_UNUSED(frameView, scrollingCoordinator->coordinatesScrollingForFrameView(frameView));
 
-        ScrollingNodeID nodeID = attachScrollingNode(layer, nodeType, parentNodeID);
-        if (!nodeID)
-            return;
-            
-        LOG_WITH_STREAM(Compositing, stream << "Registering ViewportConstrained " << nodeType << " node " << nodeID << " (layer " << backing->graphicsLayer()->primaryLayerID() << ") as child of " << parentNodeID);
+        newNodeID = attachScrollingNode(*m_renderView.layer(), m_renderView.frame().isMainFrame() ? ScrollingNodeType::MainFrame : ScrollingNodeType::Subframe, treeState);
+
+        if (!newNodeID) {
+            ASSERT_NOT_REACHED();
+            return treeState.parentNodeID.valueOr(0);
+        }
 
         if (changes & ScrollingNodeChangeFlags::Layer)
-            scrollingCoordinator->setNodeLayers(nodeID, backing->graphicsLayer());
+            updateScrollingNodeLayers(newNodeID, layer, *scrollingCoordinator);
 
         if (changes & ScrollingNodeChangeFlags::LayerGeometry) {
-            switch (nodeType) {
-            case ScrollingNodeType::Fixed:
-                scrollingCoordinator->setViewportConstraintedNodeGeometry(nodeID, computeFixedViewportConstraints(layer));
-                break;
-            case ScrollingNodeType::Sticky:
-                scrollingCoordinator->setViewportConstraintedNodeGeometry(nodeID, computeStickyViewportConstraints(layer));
-                break;
-            case ScrollingNodeType::MainFrame:
-            case ScrollingNodeType::Subframe:
-            case ScrollingNodeType::FrameHosting:
-            case ScrollingNodeType::Overflow:
-                break;
-            }
+            scrollingCoordinator->setRectRelativeToParentNode(newNodeID, rootParentRelativeScrollableRect());
+            scrollingCoordinator->setScrollingNodeScrollableAreaGeometry(newNodeID, frameView);
+            scrollingCoordinator->setFrameScrollingNodeState(newNodeID, frameView);
+        }
+    } else {
+        newNodeID = attachScrollingNode(layer, ScrollingNodeType::Overflow, treeState);
+        if (!newNodeID) {
+            ASSERT_NOT_REACHED();
+            return treeState.parentNodeID.valueOr(0);
         }
         
-        parentNodeID = nodeID;
-    } else
-        detachScrollCoordinatedLayer(layer, ScrollCoordinationRole::ViewportConstrained);
-        
-    if (roles.contains(ScrollCoordinationRole::Scrolling)) {
-        if (isRenderViewLayer)
-            updateScrollCoordinationForThisFrame(parentNodeID, changes);
-        else {
-            ScrollingNodeType nodeType = ScrollingNodeType::Overflow;
-            ScrollingNodeID nodeID = attachScrollingNode(layer, nodeType, parentNodeID);
-            if (!nodeID)
-                return;
-
-            if (changes & ScrollingNodeChangeFlags::Layer)
-                scrollingCoordinator->setNodeLayers(nodeID, backing->scrollingLayer(), backing->scrollingContentsLayer());
-
-            if (changes & ScrollingNodeChangeFlags::LayerGeometry) {
-                ScrollingCoordinator::ScrollingGeometry scrollingGeometry;
-                // FIXME(https://webkit.org/b/172917): Pass parentRelativeScrollableRect?
-                scrollingGeometry.scrollOrigin = layer.scrollOrigin();
-                scrollingGeometry.scrollPosition = layer.scrollPosition();
-                scrollingGeometry.scrollableAreaSize = layer.visibleSize();
-                scrollingGeometry.contentSize = layer.contentsSize();
-                scrollingGeometry.reachableContentSize = layer.scrollableContentsSize();
-#if ENABLE(CSS_SCROLL_SNAP)
-                if (const Vector<LayoutUnit>* offsets = layer.horizontalSnapOffsets())
-                    scrollingGeometry.horizontalSnapOffsets = *offsets;
-                if (const Vector<LayoutUnit>* offsets = layer.verticalSnapOffsets())
-                    scrollingGeometry.verticalSnapOffsets = *offsets;
-                if (const Vector<ScrollOffsetRange<LayoutUnit>>* ranges = layer.horizontalSnapOffsetRanges())
-                    scrollingGeometry.horizontalSnapOffsetRanges = *ranges;
-                if (const Vector<ScrollOffsetRange<LayoutUnit>>* ranges = layer.verticalSnapOffsetRanges())
-                    scrollingGeometry.verticalSnapOffsetRanges = *ranges;
-                scrollingGeometry.currentHorizontalSnapPointIndex = layer.currentHorizontalSnapPointIndex();
-                scrollingGeometry.currentVerticalSnapPointIndex = layer.currentVerticalSnapPointIndex();
-#endif
-
-                LOG(Compositing, "Registering Scrolling scrolling node %" PRIu64 " (layer %" PRIu64 ") as child of %" PRIu64, nodeID, backing->graphicsLayer()->primaryLayerID(), parentNodeID);
+        if (changes & ScrollingNodeChangeFlags::Layer)
+            updateScrollingNodeLayers(newNodeID, layer, *scrollingCoordinator);
 
-                scrollingCoordinator->setScrollingNodeGeometry(nodeID, scrollingGeometry);
-            }
+        if (changes & ScrollingNodeChangeFlags::LayerGeometry && treeState.parentNodeID) {
+            RenderLayer* scrollingAncestorLayer = m_scrollingNodeToLayerMap.get(treeState.parentNodeID.value());
+            scrollingCoordinator->setRectRelativeToParentNode(newNodeID, parentRelativeScrollableRect(layer, scrollingAncestorLayer));
+            scrollingCoordinator->setScrollingNodeScrollableAreaGeometry(newNodeID, layer);
         }
-    } else
-        detachScrollCoordinatedLayer(layer, ScrollCoordinationRole::Scrolling);
+    }
 
-    if (roles.contains(ScrollCoordinationRole::FrameHosting)) {
-        ScrollingNodeID nodeID = attachScrollingNode(layer, ScrollingNodeType::FrameHosting, parentNodeID);
-        if (!nodeID)
-            return;
+    return newNodeID;
+}
 
-        if (changes & ScrollingNodeChangeFlags::Layer)
-            scrollingCoordinator->setNodeLayers(nodeID, backing->graphicsLayer());
+ScrollingNodeID RenderLayerCompositor::updateScrollingNodeForFrameHostingRole(RenderLayer& layer, ScrollingTreeState& treeState, OptionSet<ScrollingNodeChangeFlags> changes)
+{
+    auto* scrollingCoordinator = this->scrollingCoordinator();
 
-        LOG(Compositing, "Registering Scrolling scrolling node %" PRIu64 " (layer %" PRIu64 ") as child of %" PRIu64, nodeID, backing->graphicsLayer()->primaryLayerID(), parentNodeID);
-    } else
-        detachScrollCoordinatedLayer(layer, ScrollCoordinationRole::FrameHosting);
+    auto newNodeID = attachScrollingNode(layer, ScrollingNodeType::FrameHosting, treeState);
+    if (!newNodeID) {
+        ASSERT_NOT_REACHED();
+        return treeState.parentNodeID.valueOr(0);
+    }
+
+    if (changes & ScrollingNodeChangeFlags::Layer)
+        scrollingCoordinator->setNodeLayers(newNodeID, { layer.backing()->graphicsLayer() });
+
+    if (changes & ScrollingNodeChangeFlags::LayerGeometry && treeState.parentNodeID) {
+        RenderLayer* scrollingAncestorLayer = m_scrollingNodeToLayerMap.get(treeState.parentNodeID.value());
+        scrollingCoordinator->setRectRelativeToParentNode(newNodeID, parentRelativeScrollableRect(layer, scrollingAncestorLayer));
+    }
+
+    return newNodeID;
+}
+
+ScrollingNodeID RenderLayerCompositor::updateScrollingNodeForPositioningRole(RenderLayer& layer, ScrollingTreeState& treeState, OptionSet<ScrollingNodeChangeFlags> changes)
+{
+    auto* scrollingCoordinator = this->scrollingCoordinator();
+
+    auto newNodeID = attachScrollingNode(layer, ScrollingNodeType::Positioned, treeState);
+    if (!newNodeID) {
+        ASSERT_NOT_REACHED();
+        return treeState.parentNodeID.valueOr(0);
+    }
+
+    if (changes & ScrollingNodeChangeFlags::Layer) {
+        auto& backing = *layer.backing();
+        scrollingCoordinator->setNodeLayers(newNodeID, { backing.graphicsLayer() });
+    }
+
+    if (changes & ScrollingNodeChangeFlags::LayerGeometry && treeState.parentNodeID) {
+        // Would be nice to avoid calling computeCoordinatedPositioningForLayer() again.
+        auto positioningBehavior = computeCoordinatedPositioningForLayer(layer);
+        auto relatedNodeIDs = collectRelatedCoordinatedScrollingNodes(layer, positioningBehavior);
+        scrollingCoordinator->setRelatedOverflowScrollingNodes(newNodeID, WTFMove(relatedNodeIDs));
+
+        auto* graphicsLayer = layer.backing()->graphicsLayer();
+        LayoutConstraints constraints;
+        constraints.setAlignmentOffset(graphicsLayer->pixelAlignmentOffset());
+        constraints.setLayerPositionAtLastLayout(graphicsLayer->position());
+        constraints.setScrollPositioningBehavior(positioningBehavior);
+        scrollingCoordinator->setPositionedNodeGeometry(newNodeID, constraints);
+    }
+
+    return newNodeID;
 }
 
 ScrollableArea* RenderLayerCompositor::scrollableAreaForScrollLayerID(ScrollingNodeID nodeID) const
@@ -4081,10 +4412,8 @@ ScrollableArea* RenderLayerCompositor::scrollableAreaForScrollLayerID(ScrollingN
 
 void RenderLayerCompositor::willRemoveScrollingLayerWithBacking(RenderLayer& layer, RenderLayerBacking& backing)
 {
-    if (scrollingCoordinator()) {
-        backing.detachFromScrollingCoordinator(ScrollCoordinationRole::Scrolling);
+    if (scrollingCoordinator())
         return;
-    }
 
 #if PLATFORM(IOS_FAMILY)
     ASSERT(m_renderView.document().pageCacheState() == Document::NotInPageCache);
@@ -4092,12 +4421,13 @@ void RenderLayerCompositor::willRemoveScrollingLayerWithBacking(RenderLayer& lay
         m_legacyScrollingLayerCoordinator->removeScrollingLayer(layer, backing);
 #else
     UNUSED_PARAM(layer);
+    UNUSED_PARAM(backing);
 #endif
 }
 
+// FIXME: This should really be called from the updateBackingAndHierarchy.
 void RenderLayerCompositor::didAddScrollingLayer(RenderLayer& layer)
 {
-    updateScrollCoordinatedStatus(layer, { ScrollingNodeChangeFlags::Layer, ScrollingNodeChangeFlags::LayerGeometry });
     if (scrollingCoordinator())
         return;
 
@@ -4105,6 +4435,8 @@ void RenderLayerCompositor::didAddScrollingLayer(RenderLayer& layer)
     ASSERT(m_renderView.document().pageCacheState() == Document::NotInPageCache);
     if (m_legacyScrollingLayerCoordinator)
         m_legacyScrollingLayerCoordinator->addScrollingLayer(layer);
+#else
+    UNUSED_PARAM(layer);
 #endif
 }
 
@@ -4132,7 +4464,7 @@ void RenderLayerCompositor::setLayerFlushThrottlingEnabled(bool enabled)
     m_layerFlushTimer.stop();
     if (!m_hasPendingLayerFlush)
         return;
-    scheduleLayerFlushNow();
+    scheduleLayerFlush();
 }
 
 void RenderLayerCompositor::disableLayerFlushThrottlingTemporarilyForInteraction()
@@ -4175,7 +4507,7 @@ void RenderLayerCompositor::layerFlushTimerFired()
 {
     if (!m_hasPendingLayerFlush)
         return;
-    scheduleLayerFlushNow();
+    scheduleLayerFlush();
 }
 
 #if USE(REQUEST_ANIMATION_FRAME_DISPLAY_MONITOR)
@@ -4189,7 +4521,7 @@ RefPtr<DisplayRefreshMonitor> RenderLayerCompositor::createDisplayRefreshMonitor
 #endif
 
 #if ENABLE(CSS_SCROLL_SNAP)
-void RenderLayerCompositor::updateScrollSnapPropertiesWithFrameView(const FrameView& frameView)
+void RenderLayerCompositor::updateScrollSnapPropertiesWithFrameView(const FrameView& frameView) const
 {
     if (auto* coordinator = scrollingCoordinator())
         coordinator->updateScrollSnapPropertiesWithFrameView(frameView);
@@ -4225,15 +4557,15 @@ TextStream& operator<<(TextStream& ts, CompositingPolicy compositingPolicy)
 typedef HashMap<PlatformLayer*, std::unique_ptr<ViewportConstraints>> LayerMap;
 typedef HashMap<PlatformLayer*, PlatformLayer*> StickyContainerMap;
 
-void LegacyWebKitScrollingLayerCoordinator::registerAllViewportConstrainedLayers(RenderLayerCompositor& compositor, const HashSet<RenderLayer*>& layers)
+void LegacyWebKitScrollingLayerCoordinator::registerAllViewportConstrainedLayers(RenderLayerCompositor& compositor)
 {
-    if (!m_coordinateViewportConstainedLayers)
+    if (!m_coordinateViewportConstrainedLayers)
         return;
 
     LayerMap layerMap;
     StickyContainerMap stickyContainerMap;
 
-    for (auto* layer : layers) {
+    for (auto* layer : m_viewportConstrainedLayers) {
         ASSERT(layer->isComposited());
 
         std::unique_ptr<ViewportConstraints> constraints;
@@ -4242,7 +4574,8 @@ void LegacyWebKitScrollingLayerCoordinator::registerAllViewportConstrainedLayers
             const RenderLayer* enclosingTouchScrollableLayer = nullptr;
             if (compositor.isAsyncScrollableStickyLayer(*layer, &enclosingTouchScrollableLayer) && enclosingTouchScrollableLayer) {
                 ASSERT(enclosingTouchScrollableLayer->isComposited());
-                stickyContainerMap.add(layer->backing()->graphicsLayer()->platformLayer(), enclosingTouchScrollableLayer->backing()->scrollingLayer()->platformLayer());
+                // what
+                stickyContainerMap.add(layer->backing()->graphicsLayer()->platformLayer(), enclosingTouchScrollableLayer->backing()->scrollContainerLayer()->platformLayer());
             }
         } else if (layer->renderer().isFixedPositioned())
             constraints = std::make_unique<FixedPositionViewportConstraints>(compositor.computeFixedViewportConstraints(*layer));
@@ -4257,31 +4590,22 @@ void LegacyWebKitScrollingLayerCoordinator::registerAllViewportConstrainedLayers
 
 void LegacyWebKitScrollingLayerCoordinator::unregisterAllViewportConstrainedLayers()
 {
-    if (!m_coordinateViewportConstainedLayers)
+    if (!m_coordinateViewportConstrainedLayers)
         return;
 
     LayerMap layerMap;
     m_chromeClient.updateViewportConstrainedLayers(layerMap, { });
 }
 
-static bool scrollbarHasDisplayNone(Scrollbar* scrollbar)
-{
-    if (!scrollbar || !scrollbar->isCustomScrollbar())
-        return false;
-
-    std::unique_ptr<RenderStyle> scrollbarStyle = static_cast<RenderScrollbar*>(scrollbar)->getScrollbarPseudoStyle(ScrollbarBGPart, PseudoId::Scrollbar);
-    return scrollbarStyle && scrollbarStyle->display() == DisplayType::None;
-}
-
 void LegacyWebKitScrollingLayerCoordinator::updateScrollingLayer(RenderLayer& layer)
 {
     auto* backing = layer.backing();
     ASSERT(backing);
 
-    bool allowHorizontalScrollbar = !scrollbarHasDisplayNone(layer.horizontalScrollbar());
-    bool allowVerticalScrollbar = !scrollbarHasDisplayNone(layer.verticalScrollbar());
-    m_chromeClient.addOrUpdateScrollingLayer(layer.renderer().element(), backing->scrollingLayer()->platformLayer(), backing->scrollingContentsLayer()->platformLayer(),
-        layer.scrollableContentsSize(), allowHorizontalScrollbar, allowVerticalScrollbar);
+    bool allowHorizontalScrollbar = !layer.horizontalScrollbarHiddenByStyle();
+    bool allowVerticalScrollbar = !layer.verticalScrollbarHiddenByStyle();
+    m_chromeClient.addOrUpdateScrollingLayer(layer.renderer().element(), backing->scrollContainerLayer()->platformLayer(), backing->scrolledContentsLayer()->platformLayer(),
+        layer.reachableTotalContentsSize(), allowHorizontalScrollbar, allowVerticalScrollbar);
 }
 
 void LegacyWebKitScrollingLayerCoordinator::registerAllScrollingLayers()
@@ -4303,23 +4627,42 @@ void LegacyWebKitScrollingLayerCoordinator::unregisterAllScrollingLayers()
     for (auto* layer : m_scrollingLayers) {
         auto* backing = layer->backing();
         ASSERT(backing);
-        m_chromeClient.removeScrollingLayer(layer->renderer().element(), backing->scrollingLayer()->platformLayer(), backing->scrollingContentsLayer()->platformLayer());
+        m_chromeClient.removeScrollingLayer(layer->renderer().element(), backing->scrollContainerLayer()->platformLayer(), backing->scrolledContentsLayer()->platformLayer());
     }
 }
 
 void LegacyWebKitScrollingLayerCoordinator::addScrollingLayer(RenderLayer& layer)
 {
     m_scrollingLayers.add(&layer);
+    m_scrollingLayersNeedingUpdate.add(&layer);
 }
 
 void LegacyWebKitScrollingLayerCoordinator::removeScrollingLayer(RenderLayer& layer, RenderLayerBacking& backing)
 {
     m_scrollingLayersNeedingUpdate.remove(&layer);
-    m_scrollingLayers.remove(&layer);
+    if (m_scrollingLayers.remove(&layer)) {
+        auto* scrollContainerLayer = backing.scrollContainerLayer()->platformLayer();
+        auto* scrolledContentsLayer = backing.scrolledContentsLayer()->platformLayer();
+        m_chromeClient.removeScrollingLayer(layer.renderer().element(), scrollContainerLayer, scrolledContentsLayer);
+    }
+}
 
-    auto* scrollingLayer = backing.scrollingLayer()->platformLayer();
-    auto* contentsLayer = backing.scrollingContentsLayer()->platformLayer();
-    m_chromeClient.removeScrollingLayer(layer.renderer().element(), scrollingLayer, contentsLayer);
+void LegacyWebKitScrollingLayerCoordinator::removeLayer(RenderLayer& layer)
+{
+    removeScrollingLayer(layer, *layer.backing());
+
+    // We'll put the new set of layers to the client via registerAllViewportConstrainedLayers() at flush time.
+    m_viewportConstrainedLayers.remove(&layer);
+}
+
+void LegacyWebKitScrollingLayerCoordinator::addViewportConstrainedLayer(RenderLayer& layer)
+{
+    m_viewportConstrainedLayers.add(&layer);
+}
+
+void LegacyWebKitScrollingLayerCoordinator::removeViewportConstrainedLayer(RenderLayer& layer)
+{
+    m_viewportConstrainedLayers.remove(&layer);
 }
 
 void LegacyWebKitScrollingLayerCoordinator::didChangePlatformLayerForLayer(RenderLayer& layer)