[css3-transforms] Effect of CSS transforms on scrollable areas

Under what circumstances should CSS transforms affect the scrollable area
of a scrollable container? (Sorry, I've forgotten the exact spec
terminology.)

In all major browsers, transforms normally affect the scrollable area of a
scrollable container. However, during an animation or transition, Webkit
treats the transform as having the value it had the last time it wasn't
animated/transitioned:
http://people.mozilla.org/~roc/test_transform_scrollable_area.html
Presumably this is a performance optimization related to asynchronous
compositing.

I think browsers should behave consistently here. Should we spec the Webkit
behavior? I guess that would be something like
"Transforms affect the scrollable overflow area as expected, unless they're
subject to a CSS animation or transition in which case they affect the
scrollable overflow area as if they had their values from before they were
subject to an animation/transition."

That testcase also shows that Webkit does something similar for other CSS
purposes such as getBoundingClientRect(). Should we spec that too? If so,
how?

Rob
-- 
“You have heard that it was said, ‘Love your neighbor and hate your enemy.’
But I tell you, love your enemies and pray for those who persecute you,
that you may be children of your Father in heaven. ... If you love those
who love you, what reward will you get? Are not even the tax collectors
doing that? And if you greet only your own people, what are you doing more
than others?" [Matthew 5:43-47]