Incorporate ComputedStyle::hasTransform when diffing transform styles.

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index abd609c8f6e5c374e79065e6f3e944837b4054fa..fa777e852870cfcedc15ce69e0e3eb4eef6237ac 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -387,8 +387,13 @@ void PaintLayer::updateTransformationMatrix() {
 
 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.