useSyncExternalStore should not execute a function returned by getSnapshot

[birtles]

• Check if updating to the latest Preact version resolves the issue

Describe the bug
I was debugging an issue in Zustand over in pmndrs/zustand#1137 and it looks like the issue stems from Preact's implementation of useSyncExternalStore:

export function useSyncExternalStore(subscribe, getSnapshot) {
  const [state, setState] = useState(getSnapshot);

  // TODO: in suspense for data we could have a discrepancy here because Preact won't re-init the "useState"
  // when this unsuspends which could lead to stale state as the subscription is torn down.

  useEffect(() => {
    return subscribe(() => {
      setState(getSnapshot());
    });
  }, [subscribe, getSnapshot]);

  return state;
}

Specifically, if getSnapshot returns a function, setState will execute it as per the usual behavior of setState when passed a function.

However, the documentation for getSyncExternalStore doesn't say anything about executing a function returned from getSnapshot nor does the shim version appear to do that so I can only guess that this is a bug in Preact.

To Reproduce

Sorry, I haven't had a chance to set up a minimal repro yet.

Expected behavior

It should not execute a function returned by getSnapshot().