Paint local background colors onto foreground layer.

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index 58a6e0af7fa78625423459c75e00e5de63f499e1..2742ea643222b7750153f624c8a56a70fc791b86 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -265,6 +265,7 @@ PaintLayerPainter::PaintResult PaintLayerPainter::paintLayerContents(GraphicsCon
     bool isPaintingCompositedForeground = paintFlags & PaintLayerPaintingCompositingForegroundPhase;
     bool isPaintingCompositedBackground = paintFlags & PaintLayerPaintingCompositingBackgroundPhase;
     bool isPaintingOverflowContents = paintFlags & PaintLayerPaintingOverflowContents;
+    bool isPaintingBackgroundOntoForeground = paintFlags & PaintLayerPaintingBackgroundOntoForeground;
     // Outline always needs to be painted even if we have no visible content. Also,
     // the outline is painted in the background phase during composited scrolling.
     // If it were painted in the foreground phase, it would move with the scrolled
@@ -381,7 +382,9 @@ PaintLayerPainter::PaintResult PaintLayerPainter::paintLayerContents(GraphicsCon
             scopedPaintChunkProperties.emplace(context.getPaintController(), m_paintLayer, properties);
         }
 
-        bool shouldPaintBackground = isPaintingCompositedBackground && shouldPaintContent && !selectionOnly;
+        bool shouldPaintBackground = shouldPaintContent && !selectionOnly
+            && ((isPaintingBackgroundOntoForeground && isPaintingCompositedForeground)
+                || isPaintingCompositedBackground);
         bool shouldPaintNegZOrderList = (isPaintingScrollingContent && isPaintingOverflowContents) || (!isPaintingScrollingContent && isPaintingCompositedBackground);
         bool shouldPaintOwnContents = isPaintingCompositedForeground && shouldPaintContent;
         bool shouldPaintNormalFlowAndPosZOrderLists = isPaintingCompositedForeground;
@@ -746,7 +749,7 @@ void PaintLayerPainter::paintForegroundForFragments(const PaintLayerFragments& l
         paintForegroundForFragmentsWithPhase(PaintPhaseSelection, layerFragments, context, localPaintingInfo, paintFlags, clipState);
     } else {
         if (!RuntimeEnabledFeatures::paintOptimizationsEnabled() || m_paintLayer.needsPaintPhaseDescendantBlockBackgrounds())
-            paintForegroundForFragmentsWithPhase(PaintPhaseDescendantBlockBackgroundsOnly, layerFragments, context, localPaintingInfo, paintFlags, clipState);
+            paintForegroundForFragmentsWithPhase(PaintPhaseDescendantBlockBackgroundsOnly, layerFragments, context, localPaintingInfo, paintFlags & ~PaintLayerPaintingBackgroundOntoForeground, clipState);

[flackr, 2016/08/03 15:33:13]

I missed earlier that we don't descend into children from the above paint call,
we paint descendant backgrounds in a separate phase here. We can just remove
PaintLayerPaintingBackgroundOntoForeground here. WDYT?

[chrishtr, 2016/08/03 16:44:48]

Rename to PaintLayerPaintingRootBackgroundOntoForeground then?

[flackr, 2016/08/03 17:47:10]

Done.

         if (!RuntimeEnabledFeatures::paintOptimizationsEnabled() || m_paintLayer.needsPaintPhaseFloat())
             paintForegroundForFragmentsWithPhase(PaintPhaseFloat, layerFragments, context, localPaintingInfo, paintFlags, clipState);
         paintForegroundForFragmentsWithPhase(PaintPhaseForeground, layerFragments, context, localPaintingInfo, paintFlags, clipState);