func (u *UserInterface) isFullscreen() bool {
	if !u.isRunning() {
		panic("ui: the game must be running at isFullscreen")
	}
	return u.window.GetMonitor() != nil
}
func (u *UserInterface) isFullscreen() bool {
	if !u.isRunning() {
		panic("ui: the game must be running at isFullscreen")
	}
	return u.window.GetMonitor() != nil
}
// isFullscreen must be called from the main thread.