Node title inside layout (Layout Discovery + Field Layout)
I wasn't able to get variables inside layouts due to this bug https://www.drupal.org/project/drupal/issues/2861840
Drupal 8.5 fixed the problem.
Now, inside mytheme.theme if I put this:

function MYTHEME_preprocess_layout(&amp;$variables) {
  $request     = \Drupal::request();
  $route_match = \Drupal::routeMatch();
  $title       = \Drupal::service('title_resolver')-&gt;getTitle($request, $route_match-&gt;getRouteObject());
   $variables['node_title'] = $title;
}


I'm able to use {{ node_title }} inside layouts.