If you resize your window to be narrow, then we hit an assertion in RenderMultiColumnSet::calculateBalancedHeight(). Specifically: ASSERT(m_minSpaceShortage != RenderFlowThread::maxLogicalHeight()); If you remove this assert, everything seems to lay out correctly still (since there is an if that just bails).
vertical-lr/float-multicol also.
vertical-rl/float-multicol also Note that the layout is sub-optimal as well, but that is arguably debatable.
<rdar://problem/16656289>
This was fixed by r167602.