[css-transforms] Interaction of transforms with scrollable area

Context: https://bugzilla.mozilla.org/show_bug.cgi?id=1236386
https://bug1236386.bmoattachments.org/attachment.cgi?id=8703469

This issue's probably already been discussed but I don't know where. The
question is: how does a transformed element contribute to the scrollable
area of an "overflow:auto/scroll" ancestor? For example, does its
border-box before transformation contribute to the scrollable area?

In the referenced testcase Chrome seems to say "no" if the transformed
element is a direct child of the box, "yes" otherwise. Firefox says "yes,
but only in the vertical direction". Edge says "yes". More testing would
probably be worthwhile.

Rob
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