Extreme letterboxing can occur in Win10 with crt_emudriver when you use fast start. It's normally a problem when you load up though, not when launching apps. Still, you could google how to disable it. Also, Calamity posted up the command to restart the video driver in Win10 (Win + Ctrl + Shift + B), not sure if that helps.
You should also stop *and* disable the service called AMD External Events Utility, and check the "hide modes this monitor cannot disaply" box in the display properties dialog for your monitor, the one you can reach from the desktop. I'm not sure this last will help though.
Try those two things one step at a time, and let us know if it works! IF not, it must be the frontend