I reproduces this behavior and I confirm that this is a bug. I logged it in our database and we will try to fix it for one of the following updates - for the time being, the only workaround is to set Top and Left properties to 1 (I assume you are using OffsetElementID for positioning the window). I know that this is not a complete solution as you will again get a movement with 1 pixel, but it is the best that we could offer now. If you want to eliminate the movement at this point, the only alternative is not to use animation.
Your points were updated for bringing this problem to our attention.
the Telerik team