Basically, I went to this site and followed instructions.
The gear tab mentioned here is not available but under the "Unreadl Engine" section of the interface I found the "settings" gear, scrolled to MTG Arena, and entered this into the command line prompt
Karnscionofurza#73859
This resolved it for me, https://www.reddit.com/r/MagicArena/comments/nxqp6q/macos_users_here_is_how_you_get_fullscreen_and/
Basically, I went to this site and followed instructions.
The gear tab mentioned here is not available but under the "Unreadl Engine" section of the interface I found the "settings" gear, scrolled to MTG Arena, and entered this into the command line prompt
-force-metal -force-low-power-device -screen-fullscreen 1
voila, back to full screen mode and mouse working.