[RLS] Don't create PaintLayerCompositor layers.

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp

Index: third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
index aa5ef52254f60256d7d8963d211e8db862c0cab8..9086bc31c3632a87a71e5c147d906fddb75d37d1 100644
--- a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
@@ -354,6 +354,14 @@ static void forceRecomputeVisualRectsIncludingNonCompositingDescendants(
   }
 }
 
+GraphicsLayer* PaintLayerCompositor::parentForContentLayers() const {
+  if (m_rootContentLayer)
+    return m_rootContentLayer.get();
+  // Iframe content layers will be connected by the parent frame using
+  // attachFrameContentLayersToIframeLayer.
+  return isMainFrame() ? visualViewport().scrollLayer() : nullptr;

[bokan, 2017/03/06 15:42:30]

Is it true that !rootContentLayer implies rootLayerScrolling? If so, please add
a DCHECK.

[skobes, 2017/03/06 23:00:25]

Done.

+}
+
 void PaintLayerCompositor::updateIfNeeded() {
   CompositingUpdateType updateType = m_pendingUpdateType;
   m_pendingUpdateType = CompositingUpdateNone;
@@ -444,9 +452,10 @@ void PaintLayerCompositor::updateIfNeeded() {
       GraphicsLayerTreeBuilder().rebuild(*updateRoot, childList);
     }
 
-    if (!childList.isEmpty()) {
-      CHECK(m_rootContentLayer && m_compositing);
-      m_rootContentLayer->setChildren(childList);
+    GraphicsLayer* contentParent = parentForContentLayers();
+    if (contentParent && !childList.isEmpty()) {
+      CHECK(m_compositing);
+      contentParent->setChildren(childList);
     }
 
     applyOverlayFullscreenVideoAdjustmentIfNeeded();
@@ -692,7 +701,7 @@ void PaintLayerCompositor::frameViewScrollbarsExistenceDidChange() {
 }
 
 void PaintLayerCompositor::rootFixedBackgroundsChanged() {
-  if (!supportsFixedRootBackgroundCompositing())
+  if (!supportsFixedRootBackgroundCompositing() || !m_containerLayer)
     return;
 
   // To avoid having to make the fixed root background layer fixed positioned to
@@ -724,13 +733,16 @@ std::unique_ptr<JSONObject> PaintLayerCompositor::layerTreeAsJSON(
   ASSERT(lifecycle().state() >= DocumentLifecycle::PaintInvalidationClean ||
          m_layoutView.frameView()->shouldThrottleRendering());
 
-  if (!m_rootContentLayer)
-    return nullptr;
-
   // We skip dumping the scroll and clip layers to keep layerTreeAsText output
   // similar between platforms (unless we explicitly request dumping from the
   // root.
   GraphicsLayer* rootLayer = m_rootContentLayer.get();
+  if (!rootLayer)
+    rootLayer = rootGraphicsLayer();
+
+  if (!rootLayer)
+    return nullptr;
+
   if (flags & LayerTreeIncludesRootLayer) {
     if (isMainFrame()) {
       while (rootLayer->parent())
@@ -768,6 +780,7 @@ bool PaintLayerCompositor::attachFrameContentLayersToIframeLayer(
   if (!layer->hasCompositedLayerMapping())
     return false;
 
+  DisableCompositingQueryAsserts disabler;
   layer->compositedLayerMapping()->setSublayers(
       GraphicsLayerVector(1, innerCompositor->rootGraphicsLayer()));
   return true;
@@ -796,11 +809,11 @@ PaintLayer* PaintLayerCompositor::rootLayer() const {
 }
 
 GraphicsLayer* PaintLayerCompositor::rootGraphicsLayer() const {
-  return m_overflowControlsHostLayer.get();
-}
-
-GraphicsLayer* PaintLayerCompositor::frameScrollLayer() const {
-  return m_scrollLayer.get();
+  if (m_overflowControlsHostLayer)
+    return m_overflowControlsHostLayer.get();
+  if (CompositedLayerMapping* clm = rootLayer()->compositedLayerMapping())
+    return clm->childForSuperlayers();
+  return nullptr;
 }
 
 GraphicsLayer* PaintLayerCompositor::scrollLayer() const {
@@ -810,14 +823,6 @@ GraphicsLayer* PaintLayerCompositor::scrollLayer() const {
   return nullptr;
 }
 
-GraphicsLayer* PaintLayerCompositor::containerLayer() const {
-  return m_containerLayer.get();
-}
-
-GraphicsLayer* PaintLayerCompositor::rootContentLayer() const {
-  return m_rootContentLayer.get();
-}
-
 void PaintLayerCompositor::setIsInWindow(bool isInWindow) {
   if (!staleInCompositingMode())
     return;
@@ -1128,7 +1133,11 @@ void PaintLayerCompositor::ensureRootLayer() {
   if (isMainFrame())
     visualViewport().createLayerTree();
 
-  if (!m_rootContentLayer) {
+  // When RLS is enabled, none of the PLC GraphicsLayers exist.
+  bool shouldCreateOwnLayers =
+      !RuntimeEnabledFeatures::rootLayerScrollingEnabled();
+
+  if (shouldCreateOwnLayers && !m_rootContentLayer) {
     m_rootContentLayer = GraphicsLayer::create(this);
     IntRect overflowRect = m_layoutView.pixelSnappedLayoutOverflowRect();
     m_rootContentLayer->setSize(
@@ -1138,7 +1147,7 @@ void PaintLayerCompositor::ensureRootLayer() {
         DOMNodeIds::idForNode(m_layoutView.node()));
   }
 
-  if (!m_overflowControlsHostLayer) {
+  if (shouldCreateOwnLayers && !m_overflowControlsHostLayer) {
     ASSERT(!m_scrollLayer);
     ASSERT(!m_containerLayer);
 
@@ -1183,9 +1192,6 @@ void PaintLayerCompositor::ensureRootLayer() {
 }
 
 void PaintLayerCompositor::destroyRootLayer() {
-  if (!m_rootContentLayer)

[bokan, 2017/03/06 15:42:30]

It seems like the existence of rootContentLayer was previously used to tell if
the PLC has been initialised. Was this just an optimisation that doesn't affect
correctness in all these cases? Does the PLC under RLS no longer maintain any
real state that we don't have to ensure it's been initialised?

[skobes, 2017/03/06 23:00:25]

PLC still tracks its state in m_rootLayerAttachment.  I think the
!m_rootContentLayer checks were unnecessary.

-    return;
-
   detachRootLayer();
 
   if (m_layerForHorizontalScrollbar) {
@@ -1225,8 +1231,7 @@ void PaintLayerCompositor::destroyRootLayer() {
 }
 
 void PaintLayerCompositor::attachRootLayer(RootLayerAttachment attachment) {
-  if (!m_rootContentLayer)
-    return;
+  DisableCompositingQueryAsserts disabler;

[szager1, 2017/03/06 22:07:47]

Why is this now necessary?

[skobes, 2017/03/06 23:00:25]

Done.

 
   // In Slimming Paint v2, PaintArtifactCompositor is responsible for the root
   // layer.
@@ -1256,7 +1261,7 @@ void PaintLayerCompositor::attachRootLayer(RootLayerAttachment attachment) {
 }
 
 void PaintLayerCompositor::detachRootLayer() {
-  if (!m_rootContentLayer || m_rootLayerAttachment == RootLayerUnattached)
+  if (m_rootLayerAttachment == RootLayerUnattached)
     return;
 
   switch (m_rootLayerAttachment) {
@@ -1266,7 +1271,7 @@ void PaintLayerCompositor::detachRootLayer() {
       // frame's layoutObject in the parent document.
       if (m_overflowControlsHostLayer)
         m_overflowControlsHostLayer->removeFromParent();
-      else
+      else if (m_rootContentLayer)

[bokan, 2017/03/06 15:42:29]

Just some questions for my own benefit: how does this work for the non-RLS case?
When is there a rootContentLayer but no overflowControlsHostLayer? If there's no
overflowControlsHostLayer, does this just disconnect the RootContentLayer from
the FrameScroll layer or is there no FrameScroll layer in that case either?

[skobes, 2017/03/06 23:00:25]

Good question.  It looks like this branch is a historical artifact - it's no
longer possible to have m_rootContentLayer without m_overflowControlsHostLayer. 
Updated.

         m_rootContentLayer->removeFromParent();
 
       if (HTMLFrameOwnerElement* ownerElement =