For some reason the menu causes the video to "flicker" on Safari Mac. It seems as if the video and the menu are racing for the highest z-index while you scroll the menu. You can see this on the elephants dream movie by going into the "Languages" menu of CC or by going to the download menu. The video has to be playing for the effect to be visible. The effect does not present on Firefox, Chrome for Mac crashes for me on mwEmbedplayer, possibly because <video> support on Chrome for Mac is unfinished.
First I thought this was a bug in webkit, or in the ogg/video module, but I created a testcase with a scrollable element on top of a video element, and it worked just fine. Thus it has to be something more specific to the dom/css/JS events of the mwEmbed player.
Possible causes, mouse events triggering a function that does something undesireable, broken z-index values.
Version: unspecified
Severity: normal
OS: Mac OS X 10.6
Platform: Macintosh
URL: http://commons.wikimedia.org/wiki/File:Elephants_Dream.ogg