Hi Marcel,
You need to set the Silverlight plug-in in windowless mode:
- In your aspx, you should set:
<asp:Silverlight Windowless="true" runat="server" ...
- If you are using HTML object, you should add this parameter:
<param name="windowless" value="true" />
Please, check this blog post, that might be helpful:
http://blogs.telerik.com/valerihristov/posts/09-06-01/reusing_and_customizing_a_single_contextmenu_on_a_treeview_with_radcontrols_for_silverlight.aspx
Sincerely yours,
Valeri Hristov
the Telerik team