FS.addEventListener(MouseEvent.CLICK, fullscreen);

// Toggles between fullscreen and normal
function fullscreen(event:MouseEvent):void
{
    if (stage.displayState == StageDisplayState.NORMAL) // If normal, full screen
    {
        stage.displayState=StageDisplayState.FULL_SCREEN;
    }
    else
    {
        stage.displayState=StageDisplayState.NORMAL; // If already fullscreen, go to normal
    }
}