User password reset form should use a more precise cache context

IS seems to be incomplete.

Proposed solution is missing (MR coming isn't valid)
User Interface mentions performance improvements, wrong section but how does this improve performance?

Thanks for the review, I updated the IS.

MR actually needs to point to 11.x please

If you are another contributor eager to jump in, please allow the previous poster at least 48 hours to respond to feedback first, so they have the opportunity to finish what they started!

Hmm, looks like the original code (not your MR) is more broken than just what's reported here.

The statement:

    // THIS CHECK MEANS WE NEED TO VARY BY user.roles:authenticated REGARDLESS OF OUTCOME
    if ($user->isAuthenticated()) {

    else {

    }

First section of that statement:

      // THIS REQUIRES THE USER TO BE A CACHEABLE DEPENDENCY
      $form['name']['#value'] = $user->getEmail();

Please note forms didn't used to be cacheable, but now they are (somewhat). So that might explain why this wasn't set in the past.

Second section of that statement (goal of this issue):

     // THIS CHECK MEANS WE NEED TO VARY BY url.query_args REGARDLESS OF OUTCOME, BUT ONLY HERE. NOT REGARDLESS OF IF-STATEMENT OUTCOME.
      $form['name']['#default_value'] = $this->getRequest()->query->get('name');

So the full code should probably look something like this:

    // Allow logged in users to request this also.
    $cacheability = (new CacheableMetadata())->addCacheContexts(['user.roles:authenticated']);
    $user = $this->currentUser();
    if ($user->isAuthenticated()) {
      $cacheability->addCacheableDependency($user);
      $form['name']['#type'] = 'value';
      $form['name']['#value'] = $user->getEmail();
      $form['mail'] = [
        '#prefix' => '<p>',
        '#markup' => $this->t('Password reset instructions will be mailed to %email. You must log out to use the password reset link in the email.', ['%email' => $user->getEmail()]),
        '#suffix' => '</p>',
      ];
    }
    else {
      $form['mail'] = [
        '#prefix' => '<p>',
        '#markup' => $this->t('Password reset instructions will be sent to your registered email address.'),
        '#suffix' => '</p>',
      ];
      $form['name']['#default_value'] = $this->getRequest()->query->get('name');
      $cacheability->addCacheContexts(['url.query_args']);
    }
    $cacheability->applyTo($form);
    $form['actions'] = ['#type' => 'actions'];
    $form['actions']['submit'] = ['#type' => 'submit', '#value' => $this->t('Submit')];

    return $form;