.tip

An effect that adds a listener without returning a cleanup adds another one on every re-run, and the old ones keep firing against unmounted components.

useEffect(() => {
  const onResize = () => setWidth(window.innerWidth);
  window.addEventListener("resize", onResize);
  return () => window.removeEventListener("resize", onResize);
}, []);

Strict mode mounts, unmounts and remounts on purpose in development so this bug shows up on the first render instead of after an hour.