Windowed full-screen is preferable because it allows quick alt-tabbing without hiding the game display (which is nice with two monitors). How is the setting enabled?
I never played LoL, but, according to a quick google search, there are two ways:
- Run the game in windowed mode, set the resolution so that it matches exactly your desktop resolution, then hide the window borders using ShiftWindow.
Config\game.cfgand you should have no need for ShiftWindow. (source)
Again, I never played League of Legends, I just googled this information and have no idea if any of this will work.