In Ubuntu 13.04, the top Unity panel shadow is displayed on top of full-screen videos / windows. The bug has been fixed a while back but it hasn't landed in Ubuntu 13.04 yet so here are two fixes / workarounds you can use until then.
|Shadow displayed on top of full-screen video|
The first workaround is very simple but it's only useful for videos (the window is unfocused so if you need to type something inside the full-screen app, it won't work, thus it's not very useful for Firefox, text editors, etc.): when playing a video in full-screen, press the Super (Windows logo) key or the Alt key and the shadow displayed at the top of the video should disappear. That's all you have to do, however, remember that you must do this each time you play a full-screen video until this is fixed in Unity.
The second workaround removes the panel shadow (even when there are no full-screen windows) and it works with any full-screen window. To apply this work-around, use the following command in a terminal:
sudo dpkg-divert --local --rename --divert /usr/share/unity/6/panel-shadow.png_disabled --add /usr/share/unity/6/panel-shadow.png
Then, you need to restart Unity so either press ALT + F2 and enter: "unity" or log out and log back in.
Workaround thanks to bwat47.
If later on you want to revert this change and get the Unity panel shadow back, use the following command:
sudo dpkg-divert --local --rename --remove /usr/share/unity/6/panel-shadow.png
And once again, restart Unity by pressing ALT + F2 and entering "unity" or log out.