Fallout 3 has a notorious bug: if you alt tab, it makes it so the game cannot be re-maximized; then you have to force close the game and re-launch it. A workaround is the Fake fullscreen mod https://www.nexusmods.com/fallout3/mods/16001/? How it works is Fallout_Fullscreen.exe is run instead of Fallout3.exe But because I use Mod Organizer for other mods, can it run Fallout_Fullscreen.exe instead of Fallout3.exe?