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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 158 matching lines @@
 CompositedLayerMapping::CompositedLayerMapping(RenderLayer& layer)
     : m_owningLayer(layer)
     , m_artificiallyInflatedBounds(false)
     , m_isMainFrameRenderViewLayer(false)
     , m_requiresOwnBackingStoreForIntrinsicReasons(true)
     , m_requiresOwnBackingStoreForAncestorReasons(true)
     , m_canCompositeFilters(false)
     , m_backgroundLayerPaintsFixedRootBackground(false)
     , m_needToUpdateGraphicsLayer(false)
     , m_needToUpdateGraphicsLayerOfAllDecendants(false)
+    , m_scrollingContentsAreEmpty(false)
 {
     if (layer.isRootLayer() && renderer()->frame()->isMainFrame())
         m_isMainFrameRenderViewLayer = true;
 
     createPrimaryGraphicsLayer();
 }
 
 CompositedLayerMapping::~CompositedLayerMapping()
 {
     // Hits in compositing/squashing/squash-onto-nephew.html.
@@ skipping 855 matching lines @@
             paintPhase |= GraphicsLayerPaintForeground;
         m_scrollingContentsLayer->setPaintingPhase(paintPhase);
     }
 }
 
 void CompositedLayerMapping::updateContentsRect()
 {
     m_graphicsLayer->setContentsRect(pixelSnappedIntRect(contentsBox()));
 }
 
+void CompositedLayerMapping::updateScrollingContentsForSelectionGaps(const IntRect& blockSelectionGapsBounds)
+{
+    if (!m_scrollingContentsAreEmpty || !m_scrollingContentsLayer)
+        return;
+
+    if (blockSelectionGapsBounds.isEmpty()) {
+        m_scrollingContentsLayer->setDrawsContent(false);
+        return;
+    }
+
+    m_scrollingContentsLayer->setDrawsContent(true);
+    m_scrollingContentsLayer->setContentsRect(IntRect());

[Ian Vollick, 2014/06/03 17:11:47]

It's weird that you can pass the empty rect. Can you find out what's going on
here? And if you truly don't use the blockSelectionGapsBounds (and just check if
they're empty), please pass a meaningful enumerator rather than a rect.

[hartmanng, 2014/06/04 15:32:32]

It turns out that the contents rect isn't being used at all in my test cases.
From comments in GraphicsLayer.h, I think the contents rect is only used for
images and videos. So setting the contents rect size has no effect on the size
of the cc layer created for background-color, highlight gaps, regular children,
etc.

afaict, the only way to minimize the size of the m_scrollingContentsLayer's main
cc layer is to use setPosition() and setSize() (ala
updateScrollingLayerGeometry()). I think I've got the size/position of the layer
itself working, but that adds an extra complication - the painting code works
relative to the composited layer. So to make the selection gaps paint in the
right location in the minimized m_scrollingContentsLayer, I think I'll have to
plumb through an extra offset to the painting code somewhere to undo the
m_scrollingContentsLayer->setPosition(). Wdyt?

[hartmanng, 2014/06/05 15:59:37]

Done (in patch set 7).

+}
+
 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_scrollingContentsAreEmpty = !hasScrollingPaintedContent;
+
+        IntRect gapRect = owningLayer().blockSelectionGapsBounds();
+        bool shouldDrawContent = !m_scrollingContentsAreEmpty || !gapRect.isEmpty();
+        m_scrollingContentsLayer->setDrawsContent(shouldDrawContent);
+        if (shouldDrawContent && m_scrollingContentsAreEmpty)
+            m_scrollingContentsLayer->setContentsRect(IntRect());
         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