Forces stacking context for computed value of transform-style:preserve-3d

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 1189 matching lines @@
     }
   }
   return false;
 }
 
 void ComputedStyle::updateIsStackingContext(bool isDocumentElement,
                                             bool isInTopLayer) {
   if (isStackingContext())
     return;
 
+  // Used value of transform-style other than 'flat' already forces stacking

[chrishtr, 2016/11/29 04:54:02]

Suggest rephrasing for clarity:

"Force a stacking context for transform-style: preserve-3d.
This happens even if preserves-3d is ignored due to 
a 'grouping property' being present which requires flattening.
See ComputedStyle::usedTransformStyle3D() and
ComputedStyle::hasGroupingProperty().

This is legacy behavior that is left ambiguous in the official specs.
See crbug.com/663650 for more details."

[trchen, 2016/11/30 01:26:23]

Done.

+  // context. This line checks the computed value in addition to used value to
+  // match behavior prior to M53. More info at crbug.com/663650 .
+  if (transformStyle3D() == TransformStyle3DPreserve3D) {
+    setIsStackingContext(true);
+    return;
+  }
+
   if (isDocumentElement || isInTopLayer || styleType() == PseudoIdBackdrop ||
       hasOpacity() || hasTransformRelatedProperty() || hasMask() ||
       clipPath() || boxReflect() || hasFilterInducingProperty() ||
       hasBackdropFilter() || hasBlendMode() || hasIsolation() ||
       hasViewportConstrainedPosition() ||
       hasPropertyThatCreatesStackingContext(willChangeProperties()) ||
       containsPaint()) {
     setIsStackingContext(true);
   }
 }
@@ skipping 1225 matching lines @@
     if (value < 0)
       fvalue -= 0.5f;
     else
       fvalue += 0.5f;
   }
 
   return roundForImpreciseConversion<int>(fvalue / zoomFactor);
 }
 
 }  // namespace blink