This is indeed a problem. The issue at play is that whenever you try to open the editor on a class that belongs to a `hidden' cluster it is not part of {CONF_LIBRARY}.classes (because hidden) even though {CONF_LIBRARY}.classes_set is True.

I think that we should not completely hide the class from `classes' but possibly let them in. However, we could change the callers of `classes' to verify that the class is not hidden from them. It seems to me (naively maybe) that we only need to change UNIVERSE_I.class_named to get the expected behavior without breaking the rest.