When watching a movie from the browser in fullscreen mode, the navigation bar at the bottom shows the URL of the file being played. When the URL is long, which is often the case with media servers, the scroll widget is reduced to a point, meaning you cannot navigate in the video at all (anyway, the whole screen is not enough to show the whole URL). Out of fullscreen, the URL is not shown, so the problem does not happen.

Most direct solution is to set max-width-chars on the URL label. But we may as well hide the URL completely, as it is really never useful for Web videos, and that's probably why it is not shown out of fullscreen.

I'm not able to make a screenshot since the bar disappears as soon as it's activated, but you can easily reproduce this using e.g. this link:
http://www.france5.fr/c-politique/index.php?page=article&numsite=4097&id_article=12215&id_rubrique=4100
(BTW, enjoy the French presidential campaign, Bastien :-p)