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..04a423e3dd88197eb77e0ac8be3b9a0454dc8aa9 100644
--- a/third_party/WebKit/Source/core/style/ComputedStyle.cpp
+++ b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
@@ -1206,6 +1206,29 @@ bool ComputedStyle::hasEntirelyFixedBackground() const
     return allLayersAreFixed(backgroundLayers());
 }
 
+static bool allLayersAreLocal(const FillLayer* layer)

[trchen, 2016/08/18 21:24:23]

Can we merge this with hasEntirelyLocalBackground() below? Or do you plan to
reuse this helper in the future?

[flackr, 2016/08/19 21:59:56]

Of course, I actually have a follow-on patch in which I merge this
(https://codereview.chromium.org/2264663002) but I may as well do this now.

[trchen, 2016/08/19 22:49:14]

Cool.

+{
+    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;
+}
+
+bool ComputedStyle::hasEntirelyLocalBackground() const
+{
+    const FillLayer& layer = backgroundLayers();
+    return allLayersAreLocal(&layer);
+}
+
 const CounterDirectiveMap* ComputedStyle::counterDirectives() const
 {
     return m_rareNonInheritedData->m_counterDirectives.get();