See https://bugzilla.mozilla.org/attachment.cgi?id=596623 for a testcase.
Gecko and Presto get this right: the scrollbar appears once there is overflow.
This is somewhat by design. We don't want to slow down accelerated transform animations by having to recompute overflow for every frame. Perhaps we could recompute overflow at the start of the transition, using the final transform value though.
If you think the spec should allow this behavior, then it should probably do so explicitly...