How to avoid "The form has become outdated" after login in separate tab
The error is related to form CSRF protection, the error message comes from core form validation.

To avoid the error, it's possible to remove that CSRF token validation check in hook_form_alter e.g:

function my_module_form_alter(&amp;$form, &amp;$form_state, $id) {
  if (strpos($id, 'webform') === 0) {
    // Bypass per user CSRF token check and allow cache.
    unset($form['#token']);
  }
}


The caveat is, the form will no longer have default CSRF protection.

Alternatively, you might be able to detect the user has logged in (e.g custom local storage set event) and reload the original page to refresh it as is.