It was a bit of puzzle how to properly enable Twig debug in Drupal 8 and set up template refresh on every load. I had to go thought quite a few tips to figure it out on #SprintWeekend instead of making patches. There are a few guides but all were yet missing a step.
Oh well, long live good old “Rebuild the theme registry on every page load” in devel module for Drupal 7. I hope that will be solved in easier way for Drupal 8 too soon.
Don’t forget, in Drupal 8 templates are also cached at file system level, not just in database as it was in Drupal 7 (or previous).