Paint local attachment backgrounds into composited scrolling contents layer

third_party/WebKit/Source/core/style/ComputedStyle.cpp

Index: third_party/WebKit/Source/core/style/ComputedStyle.cpp
diff --git a/third_party/WebKit/Source/core/style/ComputedStyle.cpp b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
index 20681097384728f6e84ccc056967ab3512b3b711..f8aebb9193715de184bbf5780ff7e0b3d6e76460 100644
--- a/third_party/WebKit/Source/core/style/ComputedStyle.cpp
+++ b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
@@ -1206,6 +1206,23 @@ bool ComputedStyle::hasEntirelyFixedBackground() const
     return allLayersAreFixed(backgroundLayers());
 }
 
+bool ComputedStyle::hasEntirelyLocalBackground() const
+{
+    const FillLayer* layer = &backgroundLayers();
+    for (; layer; layer = layer->next()) {
+        if (layer->attachment() == LocalBackgroundAttachment)
+            continue;
+        // Solid color layers with a background clip of the padding box can be treated
+        // as local.
+        // TODO(flackr): We can handle other fill boxes with solid colors as long as they
+        // are essentially the same (i.e. PaddingFillBox == ContentFillBox when padding = 0).
+        if (!layer->image() && !layer->next() && layer->clip() == PaddingFillBox)
+            continue;
+        return false;
+    }
+    return true;
+}
+
 const CounterDirectiveMap* ComputedStyle::counterDirectives() const
 {
     return m_rareNonInheritedData->m_counterDirectives.get();