When a video or iframes is flowed in an auto-height region, the region is 4px taller than the content node.
Is this a region specific issue? Have you tried to lay-out a video in an auto-height div and see what happens?
This also reproduces when the video is laid out in a plain div, too, so this is not region specific.
Changed bug description to reflect new findings, removed dependency on bug 57312.