I changed UserOptions.ini so Mode = Fullscreen but it got switched to Windowed without any action by me.
There's some kind of bug where this sometimes happens to people. It happens less when I don't touch the computer during game startup.
Happens if you tab out and back too. The built in Operating System full screen key will remedy this for you. Alt+Enter
I set mine to read only. Plus I have a safe copy. Sucks I don't have the one anymore where fog was removed.