Support subnavigation on non-page views that are included in the site navigation

[kimisgold]

Because the navigation bar doesn't do a dropdown display, it can be impossible to access pages set up in your nav. Particularly, the browse pages don't display the same subnavigation that normal pages do, even when you set it up that way in the settings. (Also true for Faceted Browse, Map Browse, and other modules that add pages.)

So, while it's technically possible for users to set up the navigation any way they want, the front-end won't actually be able to display everything.

I think the solution is to either: create error/warning messages in the admin side navigation settings based on the theme; add the subnavigation to every single browse page, including module pages; or modify the Papers navigation to show the whole hierarchy in some way.

Originally from omeka-s-themes/papers#26