What that means is that, for some reason, your game is not rendering anything, or only a limited ammount of stuff. In general, there are two places you see that effect: In a 3D view of the game when you don't have a camera, and in the 2D view when there is no GUI, not even the main menu background. Usually, you only see the 2D version when you hit F10 in the GUI editor to test the GUI you are editing, because when you do that, only that one GUI is shown, and unless it covers the whole screen, some parts of the screen will act like that. Until you go back to the GUI editor, the screen will continue to do that, even if you close the GUI you are editing. Tip: You can return to the game or the main menu from the GUI editor by opening the playGUI (in-game) or the mainMenuGUI and hitting F10.
Summary: When the mouse does that, it is completely normal and can easily be fixed, not a system-related issue.
