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..a00b8dc0649adc7f2167d35fcb253deb51677518 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -176,6 +176,7 @@ CompositedLayerMapping::CompositedLayerMapping(RenderLayer& layer)
     , m_backgroundLayerPaintsFixedRootBackground(false)
     , m_needToUpdateGraphicsLayer(false)
     , m_needToUpdateGraphicsLayerOfAllDecendants(false)
+    , m_scrollingContentsAreEmpty(false)
 {
     if (layer.isRootLayer() && renderer()->frame()->isMainFrame())
         m_isMainFrameRenderViewLayer = true;
@@ -1051,6 +1052,20 @@ 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) {
@@ -1062,7 +1077,13 @@ void CompositedLayerMapping::updateDrawsContent()
         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;
     }