How exactly are you showing this window? Are you using its OnCilentShow event to change its size (via the setSize() method)? If so you would need to re-center it again after modifying its size because the top left corner stays in place when such resizing takes place. Currently I am only guessing, but if you provide the opening function and RadWindow markup that are causing this behavior I will, in turn, be able to provide a more accurate answer.
the Telerik team
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