Doubles/Shadow bugs from STEAM.

[seVes]

Hey guys + girls,

i don't really know how to find the correct topic for this, so i make 2 pictures for describing my "problem".

It isn't really a problem, it's more like a "bug" or "misconfiguration". I don't know.

When i start STEAM (steam-for-linux, a game platform which released games for linux / windows), i got at the right side the correct panel/menu.
and left a shadow, a duplicate, or something like that. Its just drawed and not usuable, when i move a window, like lxterminal, to it, it clears the section where the lxterminal appears.

Take a look at the pictures. What is this? I got this only at steam for now.

http://imageshack.us/a/img443/9180/dsc1540z.jpg
http://imageshack.us/a/img824/3187/dsc1541wz.jpg

Do you need some package-infos? Let me know...

Thanks for help.
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Alex / seVes