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

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 a88aa09eb673ebed99c822e09b94e6bea910bead..9441ff8be8857be296e8fe1c7ea406555683c522 100644
--- a/third_party/WebKit/Source/core/style/ComputedStyle.cpp
+++ b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
@@ -1207,6 +1207,14 @@ void ComputedStyle::updateIsStackingContext(bool isDocumentElement,
   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() ||