The best way to use the RadWindow is through its client-side API
and to keep it out of any postbacks (if possible), including partial ones.
All that being said, you can give the PreserveClientState property of the RadWindowManager a shot. Note that it keeps the size and position of RadWnidows from its collection in a cookie, so it works not only for postbacks but for different sessions as well. Also, a cookie may be rejected by the client's browser.
All the best,
the Telerik team
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