[Suggestion]: Document that useFormStatus works with a preventDefault-ed onSubmit + startTransition #8682

Summary

Document that useFormStatus also reports pending: true when a submit event handler calls event.preventDefault() and starts a Transition with startTransition, not only when the form uses the action prop.

Page

https://react.dev/reference/react-dom/hooks/useFormStatus

Details

I'm implementing a custom <Form> component that accepts a Transition-based prop like submitAction (it validates and transforms the values, then calls submitAction inside startTransition). I wanted to provide the form's pending state through useFormStatus itself, instead of adding a custom context. While investigating, I found that this already works, but because it is not documented, I could not tell whether it is safe to rely on.

The useFormStatus page currently describes the status only in terms of the action prop on the parent <form>. However, since react/react#29019 (shipped in React 19.0.0), useFormStatus is also activated when startTransition is called inside a submit event handler that is preventDefault-ed:

After this change, useFormStatus will also be activated if you call startTransition(actionFn) inside a submit event handler that is preventDefault-ed.

The PR describes this as the way for UI component libraries to implement a custom action prop with onSubmit, and the behavior is covered by tests in ReactDOMForm-test.js:

• useFormStatus is activated if startTransition is called inside preventDefault-ed submit event
• useFormStatus is not activated if startTransition is not called
• useFormStatus is not activated if event is not preventDefault-ed

This matters for form components like mine that cannot use the action prop, for example because they validate and transform values before calling the action, or because they do not want the automatic form reset.

I confirmed the behavior with React 19.3.0 in Chrome.

Playground: https://codesandbox.io/p/sandbox/custom-useformstatus-fm2fkl

// App.js
import { startTransition, useActionState } from "react";
import { useFormStatus } from "react-dom";

// A fake request that takes 2 seconds.
function save(value) {
  return new Promise((resolve) => setTimeout(() => resolve(value), 2000));
}

// A form component in a UI library. It does not use the `action` prop,
// e.g. because it validates and transforms the values before calling the action.
function MyForm({ submitAction, children }) {
  return (
    <form
      onSubmit={(event) => {
        event.preventDefault();
        const value = new FormData(event.currentTarget).get("name");
        startTransition(async () => {
          await submitAction(value);
        });
      }}
    >
      {children}
    </form>
  );
}

function SubmitButton() {
  const { pending } = useFormStatus();
  return (
    <button type="submit" disabled={pending}>
      {pending ? "Sending" : "Send"}
    </button>
  );
}

function App() {
  const [state, sendAction] = useActionState(async (prev, value) => {
    await save(value);
    return value;
  }, null);
  return (
    <MyForm submitAction={sendAction}>
      <input name="name" />
      <SubmitButton />
      {state && <p>Saved: {state}</p>}
    </MyForm>
  );
}

export default App;

pending stays true until the action finishes, and the form is not reset.

Suggested changes:

• In "Returns", say that pending is also true while a Transition started from a preventDefault-ed submit event of the parent <form> is pending.
• In "Usage", add a short example of the onSubmit + preventDefault() + startTransition pattern, and note that both preventDefault() and startTransition in the same event are required.