Paint local background colors onto foreground layer.

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.cpp
index 45d65c050b8fe16887cb9cba07653e41c14699ea..2328a074e1ea273a7cefc507a184623829d9ce95 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.cpp
@@ -45,6 +45,12 @@ protected:
         return graphicsLayer->m_previousInterestRect;
     }
 
+    bool shouldPaintBackgroundOntoScrollingContentsLayer(const char* elementId)
+    {
+        CompositedLayerMapping* mapping = toLayoutBlock(getLayoutObjectByElementId(elementId))->layer()->compositedLayerMapping();
+        return mapping->shouldPaintBackgroundOntoScrollingContentsLayer();
+    }
+
 private:
     void SetUp() override
     {
@@ -626,4 +632,31 @@ TEST_F(CompositedLayerMappingTest, ScrollingContentsAndForegroundLayerPaintingPh
     EXPECT_FALSE(mapping->foregroundLayer());
 }
 
+TEST_F(CompositedLayerMappingTest, ShouldPaintBackgroundOntoScrollingContentsLayer)
+{
+    document().frame()->settings()->setPreferCompositingToLCDTextEnabled(true);
+    setBodyInnerHTML(
+        "<style>.scroller { overflow: scroll; will-change: transform; width: 300px; height: 300px;} .spacer { height: 1000px; }</style>"
+        "<div id='scroller1' class='scroller' style='background: white local;'>"
+        "    <div id='negative-composited-child' style='background-color: red; width: 1px; height: 1px; position: absolute; backface-visibility: hidden; z-index: -1'></div>"
+        "    <div class='spacer'></div>"
+        "</div>"
+        "<div id='scroller2' class='scroller' style='background: white content-box; padding: 10px;'>"
+        "    <div class='spacer'></div>"
+        "</div>"
+        "<div id='scroller3' class='scroller' style='background: white local content-box; padding: 10px;'>"
+        "    <div class='spacer'></div>"
+        "</div>"
+        );
+
+    // First scroller cannot paint background into scrolling contents layer because it has a negative z-index child.
+    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller1"));
+
+    // Second scroller cannot paint background into scrolling contents layer because it has a content-box clip without local attachment.
+    EXPECT_FALSE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller2"));
+
+    // Third scroller can paint background into scrolling contents layer.
+    EXPECT_TRUE(shouldPaintBackgroundOntoScrollingContentsLayer("scroller3"));
+}
+
 } // namespace blink