Provide the 'general' category within the plugin manager, and ensure that it remains unaltered

Could be as simple as this :)

@amateescu LGTM!

P.S.: I do think it's still slightly confusing, but this is out of scope. What do you think, @amateescu and others?

+++ b/core/lib/Drupal/Core/Field/FieldTypeCategoryManager.php
@@ -79,6 +79,17 @@ protected function getDiscovery(): YamlDiscovery {
+    if (!isset($definitions[FieldTypeCategoryManagerInterface::FALLBACK_CATEGORY])) {
+      throw new \LogicException('Missing fallback category.');

+++ b/core/modules/comment/comment.module
@@ -789,9 +789,7 @@ function comment_entity_view_display_presave(EntityViewDisplayInterface $display
+  // The `comment` field type belongs in the `general` category, so the
...
+  $definitions[FieldTypeCategoryManagerInterface::FALLBACK_CATEGORY]['libraries'][] = 'comment/drupal.comment-icon';

This is the thing that makes it confusing IMHO: the constant says "fallback", but it's really the "general" category.

Renaming the constant would make this clearer.

And we can still do that because this didn't ship in any release!🤞🤓

I think we called it a fallback category because it has some special functionality, see \Drupal\Core\Field\FallbackFieldTypeCategory. It's also not rendered like other categories. Items in this category are rendered on their own, without a category, meaning it's not a real category. If we want to consider renaming or changing it, let's do that in a follow-up issue.

Committed 1ea295c and pushed to 11.x. Thanks!