Add support for accelerated fixed root background

Source/core/rendering/RenderLayerCompositor.cpp

Index: Source/core/rendering/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/RenderLayerCompositor.cpp b/Source/core/rendering/RenderLayerCompositor.cpp
index 8dfcfe82615d49e2c7f06891b70e7eb740f02b28..3fb29cc28f77e51984e60f9802f347c6b62d04d6 100644
--- a/Source/core/rendering/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/RenderLayerCompositor.cpp
@@ -1087,6 +1087,22 @@ void RenderLayerCompositor::rebuildCompositingLayerTree(RenderLayer* layer, Vect
         // If a negative z-order child is compositing, we get a foreground layer which needs to get parented.
         if (layerBacking && layerBacking->foregroundLayer())
             childList.append(layerBacking->foregroundLayer());
+
+        // To avoid having to make the fixed root background layer fixed positioned to
+        // stay put, we position it in the layer tree as follows:
+        //
+        // + Overflow controls host
+        //   + Frame clip
+        //     + (Fixed root background) <-- Here.
+        //     + Frame scroll
+        //       + Root content layer
+        //   + Scrollbars
+        //
+        // That is, it needs to be the first child of the frame clip, the sibling of
+        // the frame scroll layer. The compositor does not own the background layer, it
+        // just positions it (like the foreground layer above).
+        if (layerBacking && layerBacking->backgroundLayer())
+            m_clipLayer->addChildBelow(layerBacking->backgroundLayer(), m_scrollLayer.get());

[jamesr, 2013/06/13 19:48:47]

why is this here? the code above and below this portion are about building up
the z-order and normal flow lists, but this comment and code have nothing to do
with that. why is it in a layer->isStackingContext() block? what does that have
to do with background layers?

look at how the overflow controls are set up, which is somewhat similar. those
only update when there's a change in overflow state and have a dedicated
function to set up the correct hierarchy and positioning. how is this case
different?

     }
 
     if (Vector<RenderLayer*>* normalFlowList = layer->normalFlowList()) {
@@ -1183,9 +1199,6 @@ void RenderLayerCompositor::frameViewDidScroll()
     }
 
     m_scrollLayer->setPosition(FloatPoint(-scrollPosition.x(), -scrollPosition.y()));
-
-    if (GraphicsLayer* fixedBackgroundLayer = fixedRootBackgroundLayer())
-        fixedBackgroundLayer->setPosition(IntPoint(frameView->scrollOffsetForFixedPosition()));
 }
 
 void RenderLayerCompositor::frameViewDidLayout()
@@ -1194,7 +1207,8 @@ void RenderLayerCompositor::frameViewDidLayout()
 
 void RenderLayerCompositor::rootFixedBackgroundsChanged()
 {
-    // FIXME: Implement when root fixed background layer is implemented.
+    if (supportsFixedRootBackgroundCompositing())
+        setCompositingLayersNeedRebuild();

[jamesr, 2013/06/13 19:48:47]

this seems really heavyweight. does promoting due to fixed background cause
other layers to need to be rebuilt?

 }
 
 void RenderLayerCompositor::scrollingLayerDidChange(RenderLayer* layer)
@@ -1220,9 +1234,13 @@ String RenderLayerCompositor::layerTreeAsText(LayerTreeFlags flags)
     if (flags & LayerTreeFlagsIncludePaintingPhases)
         layerTreeBehavior |= LayerTreeAsTextIncludePaintingPhases;
 
+    GraphicsLayer* rootLayer = m_rootContentLayer.get();
+    if (flags & LayerTreeFlagsIncludeRootLayer)
+        rootLayer = rootGraphicsLayer();
+
     // We skip dumping the scroll and clip layers to keep layerTreeAsText output
     // similar between platforms.
-    String layerTreeText = m_rootContentLayer->layerTreeAsText(layerTreeBehavior);
+    String layerTreeText = rootLayer->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.
@@ -2101,7 +2119,10 @@ void RenderLayerCompositor::paintContents(const GraphicsLayer* graphicsLayer, Gr
 
 bool RenderLayerCompositor::supportsFixedRootBackgroundCompositing() const
 {
-    return false;  // FIXME: Return true if this is supported when implemented.
+    return m_renderView->layer()
+        && m_renderView->layer()->backing()
+        && m_renderView->document()->settings()
+        && m_renderView->document()->settings()->acceleratedCompositingForFixedRootBackgroundEnabled();

[jamesr, 2013/06/13 19:48:47]

why isn't this setting stashed along with the other compositing triggers?

 }
 
 bool RenderLayerCompositor::needsFixedRootBackgroundLayer(const RenderLayer* layer) const
@@ -2112,19 +2133,6 @@ bool RenderLayerCompositor::needsFixedRootBackgroundLayer(const RenderLayer* lay
     return supportsFixedRootBackgroundCompositing() && m_renderView->rootBackgroundIsEntirelyFixed();
 }
 
-GraphicsLayer* RenderLayerCompositor::fixedRootBackgroundLayer() const
-{
-    // Get the fixed root background from the RenderView layer's backing.
-    RenderLayer* viewLayer = m_renderView->layer();
-    if (!viewLayer)
-        return 0;
-    
-    if (viewLayer->isComposited() && viewLayer->backing()->backgroundLayerPaintsFixedRootBackground())
-        return viewLayer->backing()->backgroundLayer();
-
-    return 0;
-}
-
 static void resetTrackedRepaintRectsRecursive(GraphicsLayer* graphicsLayer)
 {
     if (!graphicsLayer)