Prevent will-change:transform from changing the computed value of transform

Source/core/style/ComputedStyle.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *
  * 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
@@ skipping 863 matching lines @@
     TextEmphasisPosition textEmphasisPosition() const { return static_cast<TextEmphasisPosition>(rareInheritedData->textEmphasisPosition); }
     const AtomicString& textEmphasisMarkString() const;
 
     RubyPosition rubyPosition() const { return static_cast<RubyPosition>(rareInheritedData->m_rubyPosition); }
 
     TextOrientation textOrientation() const { return static_cast<TextOrientation>(rareInheritedData->m_textOrientation); }
 
     ObjectFit objectFit() const { return static_cast<ObjectFit>(rareNonInheritedData->m_objectFit); }
     LengthPoint objectPosition() const { return rareNonInheritedData->m_objectPosition; }
 
-    // Return true if any transform related property (currently transform/motionPath, transformStyle3D or perspective)
-    // indicates that we are transforming
-    bool hasTransformRelatedProperty() const { return hasTransform() || preserves3D() || hasPerspective(); }
+    // Return true if any transform related property (currently transform/motionPath, transformStyle3D, perspective,
+    // or will-change:transform) indicates that we are transforming. will-change:transform should result in
+    // the same rendering behavior as having a transform, including the creation of a containing block
+    // for fixed position descendants.
+    bool hasTransformRelatedProperty() const { return hasTransform() || preserves3D() || hasPerspective() || hasWillChangeTransformHint(); }
 
     enum ApplyTransformOrigin { IncludeTransformOrigin, ExcludeTransformOrigin };
     enum ApplyMotionPath { IncludeMotionPath, ExcludeMotionPath };
     enum ApplyIndependentTransformProperties { IncludeIndependentTransformProperties , ExcludeIndependentTransformProperties };
     void applyTransform(TransformationMatrix&, const LayoutSize& borderBoxSize, ApplyTransformOrigin, ApplyMotionPath, ApplyIndependentTransformProperties) const;
     void applyTransform(TransformationMatrix&, const FloatRect& boundingBox, ApplyTransformOrigin, ApplyMotionPath, ApplyIndependentTransformProperties) const;
     bool hasMask() const { return rareNonInheritedData->m_mask.hasImage() || rareNonInheritedData->m_maskBoxImage.hasImage(); }
 
     TextCombine textCombine() const { return static_cast<TextCombine>(rareInheritedData->m_textCombine); }
     bool hasTextCombine() const { return textCombine() != TextCombineNone; }
@@ skipping 75 matching lines @@
     ScrollSnapType scrollSnapType() const { return static_cast<ScrollSnapType>(rareNonInheritedData->m_scrollSnapType); }
     ScrollSnapPoints scrollSnapPointsX() const { return rareNonInheritedData->m_scrollSnap->m_xPoints; }
     ScrollSnapPoints scrollSnapPointsY() const { return rareNonInheritedData->m_scrollSnap->m_yPoints; }
     Vector<LengthPoint> scrollSnapCoordinate() const { return rareNonInheritedData->m_scrollSnap->m_coordinates; }
     LengthPoint scrollSnapDestination() const { return rareNonInheritedData->m_scrollSnap->m_destination; }
 
     const Vector<CSSPropertyID>& willChangeProperties() const { return rareNonInheritedData->m_willChange->m_properties; }
     bool willChangeContents() const { return rareNonInheritedData->m_willChange->m_contents; }
     bool willChangeScrollPosition() const { return rareNonInheritedData->m_willChange->m_scrollPosition; }
     bool hasWillChangeCompositingHint() const;
+    bool hasWillChangeTransformHint() const;
     bool subtreeWillChangeContents() const { return rareInheritedData->m_subtreeWillChangeContents; }
 
 // attribute setter methods
 
     void setDisplay(EDisplay v) { noninherited_flags.effectiveDisplay = v; }
     void setOriginalDisplay(EDisplay v) { noninherited_flags.originalDisplay = v; }
     void setPosition(EPosition v) { noninherited_flags.position = v; }
     void setFloating(EFloat v) { noninherited_flags.floating = v; }
 
     void setLeft(const Length& v) { SET_VAR(surround, offset.m_left, v); }
@@ skipping 967 matching lines @@
 }
 
 inline bool ComputedStyle::hasPseudoElementStyle() const
 {
     return noninherited_flags.pseudoBits & PSEUDO_ELEMENT_MASK;
 }
 
 } // namespace blink
 
 #endif // ComputedStyle_h