function Box() {
  const [state, setState] = useState({ left: 0, top: 0, width: 100, height: 100 });  // ...}// ...
  useEffect(() => {
    function handleWindowMouseMove(e) {      // Spreading "...state" ensures we don't "lose" width and height
      setState(state => ({ ...state, left: e.pageX, top: e.pageY }));
    }    // Note: this implementation is a bit simplified
    window.addEventListener('mousemove', handleWindowMouseMove);
    return () => window.removeEventListener('mousemove', handleWindowMouseMove);
  }, []);// ...