I think that is the expected behavior of RadWindow. This control uses a Popup in order to appear on top of all controls in your page, to get more acquainted with the expected behavior of a Popup control you can check the online reference of a WPF popup which is quite similar to the Silverlight one here
You can also tell us what exactly you are trying to achieve in order to suggest some of the other controls in our suite. Thank you for writing us and we look forward to hearing from you.
the Telerik team
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