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Fullscreen is a method of running Transformice in a way that lets the player play using the full screen rather than an 800x600 windows space. Using fullscreen can be helpful in routing trolls out of hiding spaces, for showing off, or just for covering the whole screen as may be a user's preference.
To see this option, you must have played the game for a total of at least 10 hours. Until then, you will instead see a button for the help menu.
Fully zooming in can be useful in order to corner jump as you can see the corner of the ground more easily.
Originally Transformice didn't support fullscreen, and unofficial Transformice clients had to be used to simulate it. Now there are three official ways to get full screen. Clicking the fullscreen button in-game, using the downloadable client (found on Transformice's login screen) and maximizing window, or by opening up the game's swf directly in the window.
Some user-made maps are designed to give fullscreen users an advantage, by placing cheese or mouse holes in areas that are not normally visible.
The chat command for enabling fullscreen used to be /fs but is now /fullscreen.