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 8f63184303482ada56d28a7ef616b994e2ee7867..2c30b087c7771fd357df89206a1a85259fbb2f03 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;
@@ -1056,6 +1057,16 @@ void CompositedLayerMapping::updateContentsRect()
     m_graphicsLayer->setContentsRect(pixelSnappedIntRect(contentsBox()));
 }
 
+void CompositedLayerMapping::updateScrollingContentsForSelectionGaps()
+{
+    if (!m_scrollingContentsAreEmpty || !m_scrollingContentsLayer)
+        return;
+
+    // FIXME: in this case, the scrolling contents layer doesn't need to be full size. We should position and size
+    // it more conservatively. See crbug.com/381187.
+    m_scrollingContentsLayer->setDrawsContent(m_owningLayer.rendererHasBlockSelectionGaps() == RenderLayer::LayerHasBlockSelectionGaps);
+}
+
 void CompositedLayerMapping::updateDrawsContent()
 {
     if (m_scrollingLayer) {
@@ -1067,7 +1078,11 @@ void CompositedLayerMapping::updateDrawsContent()
         m_graphicsLayer->setDrawsContent(hasNonScrollingPaintedContent);
 
         bool hasScrollingPaintedContent = m_owningLayer.hasVisibleContent() && (renderer()->hasBackground() || paintsChildren());
-        m_scrollingContentsLayer->setDrawsContent(hasScrollingPaintedContent);
+        m_scrollingContentsAreEmpty = !hasScrollingPaintedContent;
+
+        RenderLayer::PresenceOfBlockSelectionGaps hasBlockSelectionGaps = owningLayer().rendererHasBlockSelectionGaps();
+        bool shouldDrawContent = !m_scrollingContentsAreEmpty || hasBlockSelectionGaps == RenderLayer::LayerHasBlockSelectionGaps;
+        m_scrollingContentsLayer->setDrawsContent(shouldDrawContent);
         return;
     }