Incorporate ComputedStyle::hasTransform when diffing transform styles.

third_party/WebKit/Source/core/paint/PaintLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 369 matching lines @@
     box->style()->applyTransform(
         *transform, box->size(), ComputedStyle::IncludeTransformOrigin,
         ComputedStyle::IncludeMotionPath,
         ComputedStyle::IncludeIndependentTransformProperties);
     makeMatrixRenderable(*transform, compositor()->hasAcceleratedCompositing());
   }
 }
 
 void PaintLayer::updateTransform(const ComputedStyle* oldStyle,
                                  const ComputedStyle& newStyle) {
-  if (oldStyle && newStyle.transformDataEquivalent(*oldStyle))
+  // It's possible for the old and new style transform data to be equivalent
+  // while hasTransform() differs, as it checks a number of conditions aside
+  // from just the matrix, including but not limited to animation state.
+  if (oldStyle && oldStyle->hasTransform() == newStyle.hasTransform() &&

[alancutter (OOO until 2018), 2017/03/22 00:31:08]

Perhaps hasTransform() would be better named hasTransformEffect().

[wkorman, 2017/03/22 01:28:01]

Some kind of rename would definitely be helpful. Effect is sort of an overloaded
phrase.

The actual logic doesn't even seem to be transform specific, for example, it
checks hasOffset() which I think can be mutated through non-default position. I
would say the method is more like "needsPaintLayer", but I think there are many
other cases that can lead to need for a paint layer.

In any case a rename best done in a different change. Open to naming thoughts
and I can pursue as follow-on if we come up with something.

+      newStyle.transformDataEquivalent(*oldStyle)) {
     return;
+  }
 
   // hasTransform() on the layoutObject is also true when there is
   // transform-style: preserve-3d or perspective set, so check style too.
   bool hasTransform =
       layoutObject().hasTransformRelatedProperty() && newStyle.hasTransform();
   bool had3DTransform = has3DTransform();
 
   bool hadTransform = transform();
   if (hasTransform != hadTransform) {
     if (hasTransform)
@@ skipping 2820 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->enclosingLayer());
 }
 #endif