New state-settling logic causes hooks + cleanup to fire unexpectedly

[jbt]

Describe the bug

After updating to the latest version of Preact (10.8.0) I've noticed bugs occurring that indicate a hook's cleanup is running unexpectedly.

In particular, under certain circumstances, a useEffect(() => { ... }, []) (which I commonly use as a "on mount/unmount" hook) is triggering even when the component isn't being mounted/unmounted

To Reproduce

Here's an example: https://codesandbox.io/s/modest-ritchie-musy7y?file=/index.js

Operative part:

function Foo() {
  const [a, setA] = useState(false);

  useEffect(() => {
    return () => {
      alert("This should only happen when component is unmounted?");
    };
  }, []);

  setA(false);

  return <button onClick={() => setA(true)}>Hi</button>;
}

Steps to reproduce the behavior:

1. Go to the codesandbox above
2. Click on "hi"
3. Alert pops up.

Expected behavior

Unless I've misunderstood hooks behaviour, the cleanup for a useEffect with empty arguments should only ever be called when the component is being unmounted. And since no unmounting is happening in the above example, the alert should never fire.

I've confirmed that this behaviour is fine with Preact up to and including 10.7.3 and only started appearing in 10.8.0

[jbt]

Ah apologies, I only just saw #3567 was pending shortly after posting this. I've tried pulling in those changes and it appears to fix the breakage I was seeing here from 10.8.0.

[JoviDeCroock]

Thank you for testing that @jbt I was about to try that ❤️

After merging the render repetition I realised the bug 😅 it did not pop up in our tests as it's more related to browser timings and clearing to undefined rather than the previous value... not too smart on my behalf there 😅

EDIT: merged, will try to release later tonight

[jbt]

Great stuff! Can confirm that 10.8.1 fixes all the various derived issues I was seeing, nice one 🎉