How to exit fullscreen monitor playback in Premiere?
As far as I am aware, there isn't a way to disable Mercury Transmit if the viewer covers the UI, unless you set up a key binding. You can assign a keyboard shortcut to the "Enable Transmit" function in the keyboard preferences (I use ctrl+shift+' as it's similar to the maximise window shortcut). You can then use this key combination to toggle the full screen display on and off.