Yes it looks like there is a bug somewhere in program. I tried it myself and when I'm using skinless interface same thing happens, but just first time animated gif kicks in. After that everything is fine and no white background square any more.
As soon as I enable skins, everything works perfect again... one more time - people, use skins !
We didn't spen that much time for nothing, haha
Joke aside, Alwil should do something regarding this issue, since it's hardcoded into avast! code. Yes, it's small and almost unnoticebale, but it's still a small bug, cosmetic one this time...
Thanks !