Display Event Title in their relevant 'Register' tabs

Thanks I'll look into this.

Not at this time.

Posting to help any others that want the title on their registration pages.

You need to get the event instance entity then the event series entity and from there you can actually get the title.

The getTitle() method didn't work for me but you can grab it from the field value.

use Drupal\Core\Url;

/**
 * Implements hook_preprocess_field().
 */
function hook_preprocess_page_title(array &$variables) {

  $route_name = \Drupal::routeMatch()->getRouteName();

  if ($route_name === 'entity.registrant.add_form') {
    // Get event instance from url.
    $url = \Drupal::service('path.current')->getPath();
    $params = Url::fromUri("internal:" . $url)->getRouteParameters();
    $entity_type = key($params);
    $entity = \Drupal::entityTypeManager()->getStorage($entity_type)->load($params[$entity_type]);

    // Load event series instance because we cant get title from instance.
    $event = $entity->getEventSeries();
    $variables['title'] = $event->title->value . '<div class="reg-title">Registration</div>';
  }
}

Thank you all for the contributions. I had to move the code over to the recurring_events_registration submodule as it was registration specific. I also streamlined the code a little.

This has been merged into the latest dev branch for both 8.x-1.x and 2.0.x