I hope you don't mind that I make a comparison to DevExpress's Ribbon control, looking at their demo they seem to have the same problem, but their pop-up doesn't get displayed outside the parent window.
I am guessing that the pop-up width is calculated to be the width of the window minus the offset.
Would you think that it's possible to adjust the template/style of your ribbon's pop-up to do the same?
I must admit that it might be rare for a user to minimize the ribbon and then show its popup while the window is off screen, but it might happen more often with dual-screens (which is how I discovered that this is happening) where the window is displayed in the area between two screens; and when that happens, it would look a bit unprofessional to a client user which might have a similar setup.
Thank you for the reply.