CSS backdrop-filter other than 'none' should force stacking context.

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 1141 matching lines @@
 }
 
 void ComputedStyle::updateIsStackingContext(bool isDocumentElement,
                                             bool isInTopLayer) {
   if (isStackingContext())
     return;
 
   if (isDocumentElement || isInTopLayer || styleType() == PseudoIdBackdrop ||
       hasOpacity() || hasTransformRelatedProperty() || hasMask() ||
       clipPath() || boxReflect() || hasFilterInducingProperty() ||
-      hasBlendMode() || hasIsolation() || hasViewportConstrainedPosition() ||
+      hasBackdropFilter() || hasBlendMode() || hasIsolation() ||
+      hasViewportConstrainedPosition() ||
       hasPropertyThatCreatesStackingContext(willChangeProperties()) ||
       containsPaint()) {
     setIsStackingContext(true);
   }
 }
 
 void ComputedStyle::addCallbackSelector(const String& selector) {
   if (!m_rareNonInheritedData->m_callbackSelectors.contains(selector))
     m_rareNonInheritedData.access()->m_callbackSelectors.append(selector);
 }
@@ skipping 1137 matching lines @@
     if (value < 0)
       fvalue -= 0.5f;
     else
       fvalue += 0.5f;
   }
 
   return roundForImpreciseConversion<int>(fvalue / zoomFactor);
 }
 
 }  // namespace blink