Allow pages to be re-ordered by drag-and-drop on page Customizer sections

[westonruter]

This could also be how page hierarchy is managed.

Careful we don't want to duplicate existing logic for nav menu management.

[westonruter]

I think that this drag-and-drop UI should only apply if a page is not hierarchical. So a post type should be able to indicate whether it can be sorted manually but is not hierarchical. I don't believe there is currently a post type support for this. There is a page-attributes support feature that includes the page template (page-template)), page parent (parent), and sort order (ordering). These should be split out into separate features.

[westonruter]

I suggest x-ordering as the initial post type support name we can use for the ability to drag-and-drop a flat list of post types.

[westonruter]

We'll likely want to only modify the menu_order of the one post being re-sorted, to avoid having to modify all of the weights of the other posts (unless necessary). To do this, we'll need to create new posts with sparse orderings, perhaps an increment of 100 (or count(posts)) between each post.

[celloexpressions]

I think the experience would be a lot better if we can set menu order and hierarchy based on usage in menus. The biggest challenge there is deciding which menu to use, and that may require a user option. But with both posts and menus in the customizer, there's an opportunity to integrate the functionality more closely.

[westonruter]

@celloexpressions I agree. Ideally the menu_order and post_parent (#65) would be linked to however the post is used in a menu. The difficulty here, of course, is when a page is used in a nav sub menu where the parent nav menu item does not correspond to a page at all. Normally in such cases I think a page would just not have a parent at all. But if we linked post parents to nav menu items… what if instead of the Parent dropdown being a list of all posts of that given type, if instead we listed all nav menu items for all menus? Difficulty then would be what to do with a page that appears in multiple menus. I guess the Post Parent selection would largely become irrelevant and should perhaps be hidden entirely if management of the relationship is done via menus instead. The menu order field could also be removed in favor of directing the user to use nav menu items instead. But… I can also see where there would be a need to manage post ordering outside the context of a nav menu. The example which comes to mind is a “employee” custom post type, where the CEO would have menu_order of 1.