Created attachment 433003 [details]
Attached test shows that animation from scale(0) has missing backing store.
Workaround is to animate from scale(0.001)
Similar to bug 218371.
We've synthesized a 100% keyframe, but it doesn't have a transform property, so we never call addStyleToCumulativeBounds() for the scale(1) state.
When we call KeyframeEffect::computeExtentOfTransformAnimation() box.style() already reflects the first keyframe, so falling back to box.style() for the 100% keyframe doesn't work (it has scale(0) in it). We need to be able to get to the style which is not affected by keyframes.