Add support for accelerated fixed root background

Source/WebCore/rendering/RenderLayerBacking.cpp

Index: Source/WebCore/rendering/RenderLayerBacking.cpp
diff --git a/Source/WebCore/rendering/RenderLayerBacking.cpp b/Source/WebCore/rendering/RenderLayerBacking.cpp
index 5a955cc5d30cf3c0a467f29415cd59bb817adb3f..ee8d38435a6636fe64e24a0dfbb23d377b797b5f 100644
--- a/Source/WebCore/rendering/RenderLayerBacking.cpp
+++ b/Source/WebCore/rendering/RenderLayerBacking.cpp
@@ -121,17 +121,10 @@ RenderLayerBacking::RenderLayerBacking(RenderLayer* layer)
     if (layer->isRootLayer()) {
         Frame* frame = toRenderView(renderer())->frameView()->frame();
         Page* page = frame ? frame->page() : 0;
-        if (page && frame && page->mainFrame() == frame) {
+        if (page && frame && page->mainFrame() == frame)
             m_isMainFrameRenderViewLayer = true;
-
-#if PLATFORM(MAC)
-            // FIXME: It's a little weird that we base this decision on whether there's a scrolling coordinator or not.
-            if (page->scrollingCoordinator())
-                m_usingTiledCacheLayer = true;
-#endif
-        }
     }
-    
+
     createPrimaryGraphicsLayer();
 
     if (m_usingTiledCacheLayer) {
@@ -287,9 +280,6 @@ void RenderLayerBacking::createPrimaryGraphicsLayer()
     m_graphicsLayer = createGraphicsLayer(layerName);
     m_creatingPrimaryGraphicsLayer = false;
 
-    if (m_usingTiledCacheLayer)
-        m_childContainmentLayer = createGraphicsLayer("TiledBacking Flattening Layer");
-
     if (m_isMainFrameRenderViewLayer) {
         m_graphicsLayer->setContentsOpaque(true);
         m_graphicsLayer->setAppliesPageScale();
@@ -404,7 +394,7 @@ bool RenderLayerBacking::shouldClipCompositedBounds() const
     if (layerForHorizontalScrollbar() || layerForVerticalScrollbar())
         return false;
 
-    if (m_usingTiledCacheLayer)
+    if (m_usingTiledCacheLayer || m_backgroundLayerPaintsFixedRootBackground)
         return false;
 
     if (!compositor()->compositingConsultsOverlap())
@@ -782,17 +772,11 @@ void RenderLayerBacking::updateGraphicsLayerGeometry()
     if (m_backgroundLayer) {
         FloatPoint backgroundPosition;
         FloatSize backgroundSize = contentsSize;
-        if (backgroundLayerPaintsFixedRootBackground()) {
-            FrameView* frameView = toRenderView(renderer())->frameView();
-            backgroundPosition = IntPoint(frameView->scrollOffsetForFixedPosition());
-            backgroundSize = frameView->visibleContentRect().size();
-        }
         m_backgroundLayer->setPosition(backgroundPosition);
         if (backgroundSize != m_backgroundLayer->size()) {
             m_backgroundLayer->setSize(backgroundSize);
             m_backgroundLayer->setNeedsDisplay();
         }
-        m_backgroundLayer->setOffsetFromRenderer(m_graphicsLayer->offsetFromRenderer());
     }
 
     if (m_owningLayer->reflectionLayer() && m_owningLayer->reflectionLayer()->isComposited()) {
@@ -885,9 +869,6 @@ void RenderLayerBacking::updateInternalHierarchy()
         if (m_ancestorClippingLayer)
             m_ancestorClippingLayer->addChild(m_contentsContainmentLayer.get());
     }
-    
-    if (m_backgroundLayer)
-        m_contentsContainmentLayer->addChild(m_backgroundLayer.get());
 
     m_graphicsLayer->removeFromParent();
     if (m_contentsContainmentLayer)
@@ -984,7 +965,7 @@ bool RenderLayerBacking::updateClippingLayers(bool needsAncestorClip, bool needs
     }
     
     if (needsDescendantClip) {
-        if (!m_childContainmentLayer && !m_usingTiledCacheLayer) {
+        if (!m_childContainmentLayer && !m_usingTiledCacheLayer && !m_backgroundLayerPaintsFixedRootBackground) {
             m_childContainmentLayer = createGraphicsLayer("Child clipping Layer");
             m_childContainmentLayer->setMasksToBounds(true);
             layersChanged = true;
@@ -1171,17 +1152,6 @@ bool RenderLayerBacking::updateBackgroundLayer(bool needsBackgroundLayer)
             m_backgroundLayer->setPaintingPhase(GraphicsLayerPaintBackground);
             layerChanged = true;
         }
-        
-        if (!m_contentsContainmentLayer) {
-            String layerName;
-#ifndef NDEBUG
-            layerName = m_owningLayer->name() + " (contents containment)";
-#endif
-            m_contentsContainmentLayer = createGraphicsLayer(layerName);
-            m_contentsContainmentLayer->setAppliesPageScale(true);
-            m_graphicsLayer->setAppliesPageScale(false);
-            layerChanged = true;
-        }
     } else {
         if (m_backgroundLayer) {
             willDestroyLayer(m_backgroundLayer.get());
@@ -1189,13 +1159,12 @@ bool RenderLayerBacking::updateBackgroundLayer(bool needsBackgroundLayer)
             m_backgroundLayer = nullptr;
             layerChanged = true;
         }
-        if (m_contentsContainmentLayer) {
-            willDestroyLayer(m_contentsContainmentLayer.get());
-            m_contentsContainmentLayer->removeFromParent();
-            m_contentsContainmentLayer = nullptr;
-            layerChanged = true;
-            m_graphicsLayer->setAppliesPageScale(true);
-        }
+    }
+    
+    if (layerChanged) {
+        // This assumes that the background layer is only used for fixed backgrounds, which is currently a correct assumption.
+        if (renderer()->view())
+            compositor()->fixedRootBackgroundLayerChanged();
     }
     
     return layerChanged;
@@ -1339,7 +1308,7 @@ void RenderLayerBacking::updateBackgroundColor(bool isSimpleContainer)
 
 void RenderLayerBacking::updateRootLayerConfiguration()
 {
-    if (!m_usingTiledCacheLayer)
+    if (!m_usingTiledCacheLayer && !m_backgroundLayerPaintsFixedRootBackground)
         return;
 
     Color backgroundColor;
@@ -1700,7 +1669,7 @@ GraphicsLayer* RenderLayerBacking::childForSuperlayers() const
 
 bool RenderLayerBacking::paintsIntoWindow() const
 {
-    if (m_usingTiledCacheLayer)
+    if (m_usingTiledCacheLayer || m_backgroundLayerPaintsFixedRootBackground)
         return false;
 
     if (m_owningLayer->isRootLayer()) {