This is really strange behaviour. I tried to reproduce it in a sample application, but to no avail. Could you please give us some more details about your application: how you are calling the Alert window and how have you created the RadWindow, have you defined it in xaml or in a separate UserControl?
If you could reproduce this issue in a sample project and send it back to us, it would be really helpful.
Looking forward to your reply.
the Telerik team
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