Hi Luc Baeten,
Unfortunately, we cannot reproduce any problem with the appearance of RadWindow "Prometheus". What browser do you use and what version?
Please look at the following online example
that might be useful for you. Also:
1. Check if your custom css does not break the layout.
2. Note that you are using different Skins for the RadWindows. In the first example it is "Vista
" and in the second one it is the "Default
Hope this will help in solving the issue.
the Telerik team