compositing bug ?

[Pascal]

Hi,

I have a bug on one of the software I used with the compositing on. The
shadow of the window remains even if the window is not on the
foreground. I don't know if it's due to xfce or the software.

No problem with other softwares on xfce, no problem either with this
particular software with a different desktop, although, I am not sure
if they are using compositing. 

Advice welcome :)

I am using xfce 4.6 rc1

An illustration of the problem is here :
http://pascal.parois.net/public/xfce.jpg

The software I am talking about is this one:
http://www.cryst.chem.uu.nl/platon/

Cheers,
Pascal