_As soon as_ you switch sauerbraten(multiverse package) to fullscreen(Step 3, above) in Ubuntu 18.10, the system mouse pointer starts the flickering in the middle of the screen(about once a second). While moving the _game_ mouse pointer around, like in circles, the system mouse pointer ceases to flicker/interrupt the game. Once you stop moving the game mouse pointer around, the flickering of the system mouse pointer returns.
A little more info:
_As soon as_ you switch sauerbraten( multiverse package) to fullscreen(Step 3, above) in Ubuntu 18.10, the system mouse pointer starts the flickering in the middle of the screen(about once a second). While moving the _game_ mouse pointer around, like in circles, the system mouse pointer ceases to flicker/interrupt the game. Once you stop moving the game mouse pointer around, the flickering of the system mouse pointer returns.