Redmine

This is actually a follow-up patch to #30725 (as I was digging deeper into this topic).

If a plugin uses e.g. ActiveJob, ActionCable etc. or puts the mailer classes into app/mailers (Rails default) instead of app/models the current implementation only adds the fixed subdirectories controllers, helpers, models of the app directory to the auto load paths and eager load paths.
The contents of Rails.application.config.eager_load_paths will only be eager loaded if eager load is enabled, so adding the plugin directories can be done always not only if eager load is enabled.

Rails defines and detects paths that should be included in auto loading or eager loading through an abstraction:
https://github.com/rails/rails/blob/v5.2.2/railties/lib/rails/engine/configuration.rb#L37

The attached patched uses this to load all pathes you would expect from normal Rails app with the same behaviour.
This might not be 100% complete but definitely better than it is now.
I guess the assets stuff should also be handled accordingly but currently I'm not quite sure how... if I will ever find out, I'll let you know ;-)