Curious problem.
With a normal window if I drag the left side past the left edge of the screen and retrieve Application.Current.MainWindow.Top I still get the accurate top number. However, if I do the same thing but change my window to a XamRibbonWindow I always get -31 no matter the top posistion. This again only when the left side is dragged out to the left, when the window is in the screen it returns the correct top number.
This is a problem with sub window posistioning as if the user has their window dragged slightly off the left of the screen I can no longer calculate the top posistion.
Any thoughts?
I was able to reproduce the issue. This looks like a bug and should be addressed with the next hotfix. You should report the issue to the support group and reference issue 14926 so you can be notified when the hotfix is available.