Hi Steve,
I built and tried your app and was not able to duplicate the described behavior, I had to open the custom control by clicking the drop-down button - so maybe you already modified the behavior. I was using Pro 2.2 and your add-in code from yesterday afternoon. It appears that the custom control doesn't become visible until the drop-down is used.
The only time the custom control was moved from its 'design' position was when I placed the ribbon tab close to the margin of my screen. I have a multi-display system, so this includes the transition area between two screens.