Force bleed avoidance for some background image rendering

third_party/WebKit/Source/core/paint/BoxDecorationData.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/BoxDecorationData.h"
 
 #include "core/layout/LayoutBox.h"
 #include "core/style/BorderEdge.h"
 #include "core/style/ComputedStyle.h"
 #include "platform/RuntimeEnabledFeatures.h"
@@ skipping 29 matching lines @@
 } // anonymous namespace
 
 BackgroundBleedAvoidance BoxDecorationData::determineBackgroundBleedAvoidance(const LayoutBox& layoutBox)
 {
     if (layoutBox.isDocumentElement())
         return BackgroundBleedNone;
 
     if (!hasBackground)
         return BackgroundBleedNone;
 
-    if (!hasBorderDecoration || !layoutBox.style()->hasBorderRadius() || layoutBox.canRenderBorderImage()) {
+    const ComputedStyle& boxStyle = *layoutBox.style();

[jbroman, 2016/02/09 22:06:21]

nit: *layoutBox.style() equivalent to layoutBox.styleRef(), which seems to be
very slightly preferred elsewhere?

+    const bool hasBorderRadius = boxStyle.hasBorderRadius();
+    if (!hasBorderDecoration || !hasBorderRadius || layoutBox.canRenderBorderImage()) {
         if (layoutBox.backgroundShouldAlwaysBeClipped())
             return BackgroundBleedClipOnly;
+        if (backgroundColor.alpha() && boxStyle.hasBackgroundImage() && hasBorderRadius) {

[f(malita), 2016/02/09 22:07:39]

I think one can stack background layers arbitrarily, even multiple images - so
only testing the background color may not be enough.  E.g., will this work for
two stacked non-opaque images when the background color is unspecified?

Also, the background painter stops painting when it encounters the first opaque
layer
(https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...).
 So if the top image is known to be opaque, we shouldn't need this extra layer
(which is not cheap).

So maybe a better precondition would be something along these lines:

  if (has_border_radius && bg_has_multiple_layers && !top_layer_is_opaque) ...

+            // Prevent bleeding of background color behind the background image when we clip for the border radius
+            return BackgroundBleedClipLayer;
+        }
         return BackgroundBleedNone;
     }
 
-    if (borderObscuresBackgroundEdge(*layoutBox.style()))
+    if (borderObscuresBackgroundEdge(boxStyle))
         return BackgroundBleedShrinkBackground;
 
     return BackgroundBleedClipLayer;
 }
 
 } // namespace blink