The callback passed to useSignalEffect should be executed after the changes (if any) have been applied to the DOM. Otherwise, it won't be a direct replacement for useEffect.

For example, people expect this to work, even if ref appears when open switches from false to true, because useEffect waits until changes have been applied to the DOM:

useEffect(() => {
  if (open) ref.current.focus();
}, [open]);

But with the current implementation of useSignalEffect, the callback runs before the DOM changes, and therefore ref.current may not exist yet:

useSignalEffect(() => {
  if (open.value) ref.current.focus();
});

Full example in StackBlitz.

Other reactive libraries introduce a tick function (Vue, Svelte, Alpine), but I'd prefer if useSignalEffect could mimic useEffect because the familiarity and ergonomics are better. I.e., access to signals after the tick() call are not tracked.