Add support for accelerated fixed root background

Source/WebCore/rendering/RenderLayerCompositor.cpp

Index: Source/WebCore/rendering/RenderLayerCompositor.cpp
diff --git a/Source/WebCore/rendering/RenderLayerCompositor.cpp b/Source/WebCore/rendering/RenderLayerCompositor.cpp
index 68340231904d10d195019ad1de35266358fb04e5..3d09f408d6b2da475809e58878bce0e10821d395 100644
--- a/Source/WebCore/rendering/RenderLayerCompositor.cpp
+++ b/Source/WebCore/rendering/RenderLayerCompositor.cpp
@@ -1234,8 +1234,7 @@ void RenderLayerCompositor::frameViewDidLayout()
 
 void RenderLayerCompositor::rootFixedBackgroundsChanged()
 {
-    RenderLayerBacking* renderViewBacking = m_renderView->layer()->backing();
-    if (renderViewBacking && renderViewBacking->usingTiledBacking())
+    if (supportsFixedRootBackgroundCompositing())
         setCompositingLayersNeedRebuild();
 }
 
@@ -1245,6 +1244,30 @@ void RenderLayerCompositor::scrollingLayerDidChange(RenderLayer* layer)
         scrollingCoordinator->scrollableAreaScrollLayerDidChange(layer);
 }
 
+void RenderLayerCompositor::fixedRootBackgroundLayerChanged()
+{
+    GraphicsLayer* backgroundLayer = fixedRootBackgroundLayer();
+    if (!backgroundLayer)
+        return;
+
+    // FIXME: there must be a better way to insert this layer into the tree.

[jamesr, 2013/04/09 19:45:48]

yes, please figure out a better way. are the layers you want to be positioned
relative to members of RenderLayerCompositor?

+    // We'll also have to unhook it at some point.
+    GraphicsLayer* root = m_rootContentLayer.get();
+    while (root && root->parent())
+        root = root->parent();
+
+    GraphicsLayer* frameClippingLayer = root->children()[0];
+    GraphicsLayer* frameScrollingLayer = frameClippingLayer->children()[0];
+    GraphicsLayer* nonCompositedContentLayer = frameScrollingLayer->children()[0];
+
+    // ??? Would we ever want to draw in both the NCCH *and* the fixed root bg?
+    // If so, it seems like we're toast. We have to make this layer invisible
+    // so that the fixed root bg can show through.
+    nonCompositedContentLayer->setDrawsContent(false);
+
+    frameClippingLayer->addChildAtIndex(backgroundLayer, 0);
+}
+
 String RenderLayerCompositor::layerTreeAsText(LayerTreeFlags flags)
 {
     updateCompositingLayers(CompositingUpdateAfterLayout);
@@ -1268,7 +1291,11 @@ String RenderLayerCompositor::layerTreeAsText(LayerTreeFlags flags)
 
     // We skip dumping the scroll and clip layers to keep layerTreeAsText output
     // similar between platforms.
-    String layerTreeText = m_rootContentLayer->layerTreeAsText(layerTreeBehavior);
+    GraphicsLayer* layer = m_rootContentLayer.get();
+    while (layer && layer->parent())
+        layer = layer->parent();
+    //String layerTreeText = m_rootContentLayer->layerTreeAsText(layerTreeBehavior);
+    String layerTreeText = layer->layerTreeAsText(layerTreeBehavior);
 
     // The true root layer is not included in the dump, so if we want to report
     // its repaint rects, they must be included here.
@@ -2237,8 +2264,8 @@ void RenderLayerCompositor::paintContents(const GraphicsLayer* graphicsLayer, Gr
 
 bool RenderLayerCompositor::supportsFixedRootBackgroundCompositing() const
 {
-    RenderLayerBacking* renderViewBacking = m_renderView->layer()->backing();
-    return renderViewBacking && renderViewBacking->usingTiledBacking();
+    return m_renderView->document()->settings() &&
+           m_renderView->document()->settings()->acceleratedCompositingForFixedRootBackgroundEnabled();
 }
 
 bool RenderLayerCompositor::needsFixedRootBackgroundLayer(const RenderLayer* layer) const