A form uses `const [state, formAction, isPending] = useActionState(serverAction, initialState)`. A developer expects `isPending` to come from a `useState` they wrote, but they wrote none. Mechanically, what drives the component to re-render with `isPending` true and then false, and where does the new `state` come from?
const [state, formAction, isPending] = useActionState(saveAction, { ok: false });
return <form action={formAction}>{isPending ? 'Saving…' : state.ok ? 'Saved' : 'Save'}</form>;