Paint local attachment backgrounds into composited scrolling contents layer

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

 /*
  * Copyright (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 1188 matching lines @@
     }
 
     return true;
 }
 
 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();
 }
 
 CounterDirectiveMap& ComputedStyle::accessCounterDirectives()
 {
     std::unique_ptr<CounterDirectiveMap>& map = m_rareNonInheritedData.access()->m_counterDirectives;
     if (!map)
         map = wrapUnique(new CounterDirectiveMap);
@@ skipping 747 matching lines @@
         if (value < 0)
             fvalue -= 0.5f;
         else
             fvalue += 0.5f;
     }
 
     return roundForImpreciseConversion<int>(fvalue / zoomFactor);
 }
 
 } // namespace blink