Support high DPI scrollbar on top level web frame.

cc/CCLayerTreeHost.cpp

Index: cc/CCLayerTreeHost.cpp
diff --git a/cc/CCLayerTreeHost.cpp b/cc/CCLayerTreeHost.cpp
index 18786f52b4d8d263bdd91820ce425a5cf7dc8cb4..d88ee5e86f8ce650930c21a37c71d6a73394d61e 100644
--- a/cc/CCLayerTreeHost.cpp
+++ b/cc/CCLayerTreeHost.cpp
@@ -467,36 +467,39 @@ void CCLayerTreeHost::updateLayers(CCTextureUpdateQueue& queue, size_t memoryAll
     updateLayers(rootLayer(), queue);
 }
 
-static void setScale(LayerChromium* layer, float deviceScaleFactor, float pageScaleFactor)
+static void setScale(LayerChromium* layer, float deviceScaleFactor, float pageScaleFactor, bool inScrollLayer)
 {
-    if (layer->boundsContainPageScale())
+    if (!inScrollLayer || layer->boundsContainPageScale())
         layer->setContentsScale(deviceScaleFactor);
     else
         layer->setContentsScale(deviceScaleFactor * pageScaleFactor);
 }
 
-static void updateLayerScale(LayerChromium* layer, float deviceScaleFactor, float pageScaleFactor)
+static void updateLayerScale(LayerChromium* layer, float deviceScaleFactor, float pageScaleFactor, bool inScrollLayer)
 {
-    setScale(layer, deviceScaleFactor, pageScaleFactor);
+    if (layer->scrollable())
+        inScrollLayer = true;

[flackr, 2012/09/17 19:37:51]

Alexandre, is the logic for finding the root scroll layer going to be changing?
This is the same logic as findRootScrollLayer in CCLayerTreeHostImpl.cpp.

[enne (OOO), 2012/09/17 20:37:00]

I think I'd prefer to keep this complexity outside of the compositor.  If a
browser compositor creates a layer tree, it's not necessarily going to want this
"outside root scroll layer" special case.  We already have a special case flag
to avoid this behavior: it's boundsContainPageScale.

I'd prefer if NonCompositedContentHost set this boundsContainPageScale flag on
itself and (recursively) all layers outside of the scroll layer, like
https://bugs.webkit.org/show_bug.cgi?id=77251 used to do for setting a different
flag on scrollbar layers.

That way the compositor doesn't have to know about this special case, and the
NonCompositedContentHost can contain all of this special case stuff about root
layers and root scrollbars.

+
+    setScale(layer, deviceScaleFactor, pageScaleFactor, inScrollLayer);
 
     LayerChromium* maskLayer = layer->maskLayer();
     if (maskLayer)
-        setScale(maskLayer, deviceScaleFactor, pageScaleFactor);
+        setScale(maskLayer, deviceScaleFactor, pageScaleFactor, inScrollLayer);
 
     LayerChromium* replicaMaskLayer = layer->replicaLayer() ? layer->replicaLayer()->maskLayer() : 0;
     if (replicaMaskLayer)
-        setScale(replicaMaskLayer, deviceScaleFactor, pageScaleFactor);
+        setScale(replicaMaskLayer, deviceScaleFactor, pageScaleFactor, inScrollLayer);
 
     const Vector<RefPtr<LayerChromium> >& children = layer->children();
     for (unsigned int i = 0; i < children.size(); ++i)
-        updateLayerScale(children[i].get(), deviceScaleFactor, pageScaleFactor);
+        updateLayerScale(children[i].get(), deviceScaleFactor, pageScaleFactor, inScrollLayer);
 }
 
 void CCLayerTreeHost::updateLayers(LayerChromium* rootLayer, CCTextureUpdateQueue& queue)
 {
     TRACE_EVENT0("cc", "CCLayerTreeHost::updateLayers");
 
-    updateLayerScale(rootLayer, m_deviceScaleFactor, m_pageScaleFactor);
+    updateLayerScale(rootLayer, m_deviceScaleFactor, m_pageScaleFactor, false);
 
     LayerList updateList;