Progress bar CSS library (core/drupal.progress) is not always applied

I worked around this issue by explicitly attaching the library in my build array:

$form['progress'] = [
  '#theme' => 'progress_bar',
  '#label' => $this->steps()[$step]['label'],
  '#percent' => $this->steps()[$step]['progress'],
  '#message' => $this->steps()[$step]['message'],
];

// Fix issue with progress library not being added in some contexts (eg:
// the intake quick form for anonymous users).
$form['progress']['#attached']['library'][] = 'core/drupal.progress';

I opened a merge request that fixes this issue.

I assume this will need tests, but marking as "Needs review" to get thoughts from the maintainers. Is this the right way to solve this?

If we can help it think we don't want to go the twig route as this fix won't appear on custom or contrib themes without updating all templates. If it's missing should attach to the render array if possible.

@smustgrave I wondered that too, before eventually found that it was already being added to various core twig templates (although it seems like the Claro one is just broken, because it references claro/classy.progress which does not exist):

• https://git.drupalcode.org/project/drupal/-/blob/e09b671ee2ad91b55e4b68f...
• https://git.drupalcode.org/project/drupal/-/blob/e09b671ee2ad91b55e4b68f...
• https://git.drupalcode.org/project/drupal/-/blob/e09b671ee2ad91b55e4b68f...

Although in all of those cases, it is adding a more specific library that extends core/drupal.progress (via libraries-extend / https://www.drupal.org/node/2497313 → )

Notably it is NOT added to the other two twig templates:

• https://git.drupalcode.org/project/drupal/-/blob/e09b671ee2ad91b55e4b68f...
• https://git.drupalcode.org/project/drupal/-/blob/e09b671ee2ad91b55e4b68f...

So you're probably right - it should be attached to the render array at a lower level.

The progress_bar theme element is defined here: https://git.drupalcode.org/project/drupal/-/blob/e09b671ee2ad91b55e4b68f...

Is it possible to attach libraries there? I don't see any other examples of that in the file, nor in hook_theme() documentation generally (https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21...).

Would we need to add a template_preprocess_progress_bar?

This is why I went the route of adding it to the twig templates - to avoid that. But maybe it's the best route?

If the claro one doesn't exist then yea that would have to be fixed at the twig level. I would definitely try template_preprocess_progress_bar