I have replied your support ticket. For convenience I am pasting my reply below:
To avoid that, you should cancel the original postback - OnClientClick ="fnOpenContractSelection(); return false;
Another thing that I noticed is that you are trying to send a custom argument to the server (as a second parameter in the __doPostback() function) - note that in this case, you would get an event validation error because of the ASP.NET 2x / 3x security. To avoid that you should either set EnablePageValidation to false for that page, or use Page.ClientScript.RegisterForEventValidation()
Please note however that this issue is not related to the RadWindow control - you will get the same problem with any other standard controls.
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