After using the Multiplex window to turn on full screen the mpv window fills your screen, and there's no obvious way to get out of full screen again. You have to use Alt+Tab to go back to the Multiplex window and click the button again. This can cause confusion. It would be great if the regular ways to toggle full screen would be available while the mpv window has focus: Double click, F, and F11. In addition to Esc to go out of full screen.
Thanks again!