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..dcb825b3548ea96a104283ef331a8875307f1e51 100644
--- a/third_party/WebKit/Source/core/style/ComputedStyle.cpp
+++ b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
@@ -1207,6 +1207,17 @@ void ComputedStyle::updateIsStackingContext(bool isDocumentElement,
   if (isStackingContext())
     return;
 
+  // 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."
+  if (transformStyle3D() == TransformStyle3DPreserve3D) {
+    setIsStackingContext(true);
+    return;
+  }
+
   if (isDocumentElement || isInTopLayer || styleType() == PseudoIdBackdrop ||
       hasOpacity() || hasTransformRelatedProperty() || hasMask() ||
       clipPath() || boxReflect() || hasFilterInducingProperty() ||