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..290bd6286983e64f5a0a4a5275032bf11104923f 100644
--- a/third_party/WebKit/Source/core/style/ComputedStyle.cpp
+++ b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
@@ -1206,6 +1206,24 @@ bool ComputedStyle::hasEntirelyFixedBackground() const
     return allLayersAreFixed(backgroundLayers());
 }
 
+static bool allLayersAreLocal(const FillLayer& layer)

[chrishtr, 2016/08/12 17:03:00]

Seems to be cleaner to inline this in hasEntirelyLocalBackground, by just
declaring


const FillLayer& layer = backgroundLayers();

at the beginning.

[flackr, 2016/08/16 17:52:51]

Done.

[trchen, 2016/08/17 01:22:55]

Did you forget to git add?

+{
+    for (const FillLayer* currLayer = &layer; currLayer; currLayer = currLayer->next()) {
+        // 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 (currLayer->attachment() != LocalBackgroundAttachment
+            && (currLayer->image() || currLayer->clip() != PaddingFillBox))

[chrishtr, 2016/08/12 17:03:00]

I don't understand the PaddingFillBox parts, could you explain it to me?

[trchen, 2016/08/13 00:06:42]

I think he is making an exception for solid color layers here. Even if a layer
is not locally attached, but if it is solid color filling the whole padding box,
it behaves no differently from a local solid color layer.

I think it would be clearer to write it this way:
for (const FillLayer* currLayer = &layer; currLayer; currLayer =
currLayer->next()) {
    if (currLayer->attachment() == LocalBackgroundAttachment)
        continue;
    if (!currLayer->image() && currLayer->clip() == PaddingFillBox)
        continue;
    return false;
}
Human brains are designed to reason DNF form easier. :)

[flackr, 2016/08/16 17:52:51]

Done.

+            return false;
+    }
+
+    return true;
+}
+
+bool ComputedStyle::hasEntirelyLocalBackground() const
+{
+    return allLayersAreLocal(backgroundLayers());
+}
+
 const CounterDirectiveMap* ComputedStyle::counterDirectives() const
 {
     return m_rareNonInheritedData->m_counterDirectives.get();