In the new sign in flow (see Issue 469801), after the user has entered their credentials, we should keep the modal dialogue around and show a spinner until the sign in has actually completed. Then the sync confirmation dialogue will be shown immediately after the previous dialogue is closed, which will prevent weird edge cases with slow connections where the confirmation dialogue gets shown a while after the previous dialogue is closed.

If possible, we should also have an "x" button in the dialogue while the spinner is shown, in case the connection is lost, so the user can abort the sign in if necessary.