I have tried to reproduce the issue, but with no luck. Attached you can find a video and a simple project where I have tried to reproduce it. I have seen that you are using a Vista theme, can you please tell me how do you set the theme? Also can you confirm that you are using a WPF 4.0 with 2012.1.326 version of the controls? It would be great If you can isolate the problem in a simple project and send it to us.
the Telerik team
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