Allow proper highlighting on universal overflow scroll.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
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  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1043 matching lines @@
 void CompositedLayerMapping::updateDrawsContent()
 {
     if (m_scrollingLayer) {
         // We don't have to consider overflow controls, because we know that the scrollbars are drawn elsewhere.
         // m_graphicsLayer only needs backing store if the non-scrolling parts (background, outlines, borders, shadows etc) need to paint.
         // m_scrollingLayer never has backing store.
         // m_scrollingContentsLayer only needs backing store if the scrolled contents need to paint.
         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;
     }
 
     bool hasPaintedContent = containsPaintedContent();
     if (hasPaintedContent && isAcceleratedCanvas(renderer())) {
         CanvasRenderingContext* context = toHTMLCanvasElement(renderer()->node())->renderingContext();
         // Content layer may be null if context is lost.
         if (blink::WebLayer* contentLayer = context->platformLayer()) {
             Color bgColor(Color::transparent);
             if (contentLayerSupportsDirectBackgroundComposition(renderer())) {
@@ skipping 1170 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore