Hello Thierry Fierens,
We confirmed this issue is still active in our latest release - 2008 Q3 SP build 1217. It will be addressed with upcoming release 2009 Q1 due in March.
As you reported a strange problem with the bottom gap I suggest that you run the attached example to ensure us about the window's behavior. If it is convenient for you please give us more information about the theme you are using.
All the best,
the Telerik team