Firebase Web Auth return url (using firebaseui)

Consider this flow:

1. user navigates to /login
2. user clicks "sign in with [provider]"
3. redirects to provider

4. success - automatically redirects back to /login

   • at this stage, the app does not know that the authentication is "pending".

5. after successful auth, the signInSuccess callback is called - but it could take seconds before this happens.

How can I present a spinner or "authenticating" state after returning but before the auth has completed? Ideally I would like to redirect the user to /login?from_provider=google.com or another URL after returning from provider but I can’ find any setting that allows this.