Allow proper highlighting on universal overflow scroll.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

Index: Source/core/rendering/compositing/CompositedLayerMapping.cpp
diff --git a/Source/core/rendering/compositing/CompositedLayerMapping.cpp b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
index 3f3eeb5bc620195b3bcd7e04d5f38dbf2058e7f6..b807673634168f881c4b4dbd1db23d216c6875ce 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -1061,8 +1061,7 @@ void CompositedLayerMapping::updateDrawsContent()
         bool hasNonScrollingPaintedContent = m_owningLayer.hasVisibleContent() && m_owningLayer.hasBoxDecorationsOrBackground();
         m_graphicsLayer->setDrawsContent(hasNonScrollingPaintedContent);
 
-        bool hasScrollingPaintedContent = m_owningLayer.hasVisibleContent() && (renderer()->hasBackground() || paintsChildren());
-        m_scrollingContentsLayer->setDrawsContent(hasScrollingPaintedContent);
+        m_scrollingContentsLayer->setDrawsContent(true);

[Ian Vollick, 2014/05/31 01:17:36]

This might be the only reasonable way to do this, but it is a pretty big hammer;
the scrolling contents layer can be enormous. I'd really like to be convinced
that other approaches won't work before we go with this. Perhaps we could store
a bool on the CLM -- m_scrollingContentsAreEmpty, say -- and when we add/clear
block selection gaps bounds in RenderLayer, we could setDrawsContent true or
false if m_scrollingContentsAreEmpty? It would also be nice to call
setContentsRect on the scrollingContentsLayer in this case so that cc knows that
the layer only has content in the selection rect.

[hartmanng, 2014/06/02 20:23:58]

Done. However, in my patch I do setContentsRect(IntRect()), and the block
selection still appears. Maybe I'm not understanding setContentsRect()? (Or
calling it too late in the frame?)

         return;
     }