User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:18.104.22.168) Gecko/20061118 (Debian-22.214.171.124-1) Epiphany/2.14
this is a feature request for the possibility to disable shadows on panel. When panel is on top, the shadow comes accross windows present immediately below, and it looks like the window is "behind" the panel. (not really clear but I can make a screenshot).
If panel is at the bottom of the screen, the shadow is indeed useless. Maybe disabling completely shadows for panel would be a good idea, but maybe people like them, so I would be ok with an option (maybe a hidden one, though).
It seems Olivier has implemented your request in the window manager, so there should be no more shadows on the panel.