Given three nested elements A, B, and C, where C is taller than B: +-----------+ |A | | +-------+ | | |B | | | | +---+ | | | | | C | | | | | | | | | | +-------+ | | | | | | +---+ | +-----------+ If B declares `overflow-y: hidden;` and C's `transform` attribute is modified, then A expands to contain C (even though B continues to "mask" C correctly). I've attached a reduced test case that demonstrates this behavior in Firefox Nightly 27.0a1 (2013-10-22). This was first discovered in Gaia--see bug 907177. As mentioned there, we believe this regression was introduced with the following commit: http://hg.mozilla.org/mozilla-central/rev/45d9e6cd3473

@roc could you comment?