On 18.10, it seems that
Alt+LeftArrowKey
flashes a black screen then asks me to log back in to my session, which resumes as it was.
I am not sure why this is something you would want to do. On my keyboard, this is so close to Ctrl+Alt+PageUp/Down and causes me to mistakenly have to log back into my session frequently. I would like to disable this keybinding.
In older Ubuntus, this setting was here. However, in the new Settings > Keyboard, I can't seem to identify what this is attempting to shortcut to to set it to null.
Solved by rebooting as suggested in the link to the comment.