Paint local background colors onto foreground layer.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index 51159637bc5b210ce7b76982670cd289db156338..6d0d87b9e861dad3123174b393e1ce3a56d9591d 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -309,8 +309,22 @@ void CompositedLayerMapping::updateContentsOpaque()
         m_graphicsLayer->setContentsOpaque(false);
         m_backgroundLayer->setContentsOpaque(m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
     } else {
-        // For non-root layers, background is always painted by the primary graphics layer.
-        m_graphicsLayer->setContentsOpaque(m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
+        // For non-root layers, background is painted by the scrolling contents layer if all backgrounds
+        // are background attachment local, otherwise background is painted by the primary graphics layer.
+        if (hasScrollingLayer() && m_owningLayer.shouldPaintBackgroundOntoScrollingContentsLayer()) {
+            // Backgrounds painted onto the foreground are clipped by the padding box rect.
+            // TODO(flackr): This should actually check the entire overflow rect within the
+            // scrolling contents layer but since we currently only trigger this for solid
+            // color backgrounds the answer will be the same.
+            m_scrollingContentsLayer->setContentsOpaque(m_owningLayer.backgroundIsKnownToBeOpaqueInRect(toLayoutBox(layoutObject())->paddingBoxRect()));
+
+            // When we paint the background onto the scrolling contents layer we are going
+            // to leave a hole in the m_graphicsLayer where the background is so it is
+            // not opaque.
+            m_graphicsLayer->setContentsOpaque(false);
+        } else {
+            m_graphicsLayer->setContentsOpaque(m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
+        }
     }
 }
 
@@ -2426,7 +2440,7 @@ void CompositedLayerMapping::paintContents(const GraphicsLayer* graphicsLayer, G
 
     if (graphicsLayer == m_backgroundLayer.get())
         paintLayerFlags |= (PaintLayerPaintingRootBackgroundOnly | PaintLayerPaintingCompositingForegroundPhase); // Need PaintLayerPaintingCompositingForegroundPhase to walk child layers.
-    else if (compositor()->fixedRootBackgroundLayer())
+    else if (compositor()->fixedRootBackgroundLayer() && m_owningLayer.isRootLayer())
         paintLayerFlags |= PaintLayerPaintingSkipRootBackground;
 
     if (graphicsLayer == m_graphicsLayer.get()
@@ -2436,6 +2450,12 @@ void CompositedLayerMapping::paintContents(const GraphicsLayer* graphicsLayer, G
         || graphicsLayer == m_childClippingMaskLayer.get()
         || graphicsLayer == m_scrollingContentsLayer.get()) {
 
+        bool paintRootBackgroundIntoScrollingContentsLayer = m_owningLayer.shouldPaintBackgroundOntoScrollingContentsLayer();

[chrishtr, 2016/08/05 00:40:08]

Add:

DCHECK(!paintRootBackgroundIntoScrollingContentsLayer || (!m_backgroundLayer &&
!m_foregroundLayer))

[flackr, 2016/08/08 18:01:30]

Done.

+        bool shouldSkipRootBackground = paintRootBackgroundIntoScrollingContentsLayer
+            ? graphicsLayer != m_scrollingContentsLayer.get()
+            : !(graphicsLayerPaintingPhase & GraphicsLayerPaintBackground);
+        if (shouldSkipRootBackground)
+            paintLayerFlags |= PaintLayerPaintingSkipRootBackground;
         GraphicsLayerPaintInfo paintInfo;
         paintInfo.paintLayer = &m_owningLayer;
         paintInfo.compositedBounds = compositedBounds();