I am running Linux Mint, with Cinnamon 19. I accidentally pressed Super-O, and the following popup icon appeared:
Pressing Super-O again seems to 'toggle' the setting, now showing this:
I looked through all of the keyboard shortcuts listed in the keyboard preferences, but Super-O wasn't listed.
I want to find out what exactly Super-O does, and what is the default option, so I can put it back how it was?