I tested this scenario and was able to isolate the described issue. It seems that the problem is caused by the "RelativeTo=BrowserWindow" setting, as if you use another relative property everything works as expected.
I will forward this to our developers for further researching and fixing. I hope that this is now a show stopper for you.
Thank you for bringing this to our attention. I updated your Telerik point for the report.
the Telerik team