Related: issue wagtail/wagtail#3380, PRs wagtail/wagtail#3407 and wagtail/wagtail#4106.

Highlighting one design decision needed from wagtail/wagtail#4106 (comment): how should the collection hierarchy be shown to users with incomplete permissions over the whole tree?

Highlighting one design decision needed from wagtail/wagtail#4106 (comment): how should the collection hierarchy be shown to users with incomplete permissions over the whole tree?

By default the collection-chooser isn't shown when user has no permission to see a collection.

Assuming the following tree of collections:
level1
...level2
......level3

If a user has access to level 2, but not 1, my implementation shows level 2 to be the root and doesn't 'know' about level 1

I'm very keen on hearing whether my implementation is adequate to be merged upstream.
May I do a PR or should I improve the implementation/design?

Thanks @SebJansen! Have given your implementation a try, and it's all working well and behaving as I'd expect it to. I think it's fine for permissions to pass down to child collections - that's consistent with how it works for pages.

In the interface for adding/editing collections, intuitively I feel like I want to click through to the parent to add a child collection, but that's only really a minor point, and getting the initial functionality in place is more important than UI tweaks.

I noticed a couple of places that don't have the new hierarchical dropdown yet - the add/edit image views, and the group permission settings - but besides that, I think this is ready for a PR 👍

I'm glad I found this, because I'm also currently working on implementing the same thing. My code shop has been using a modified version of wagtail/wagtail#4106, patched onto a custom fork of Wagtail 2.3 for months, and it's worked great. We're now (finally) in the process of upgrading to Wagtail 2.9, and I've been tasked with building a fork of 2.9 with our Hierarchical Collections functionality.

But with gasman saying that SebJansen's code looks good, I figure it'll be better to work with his implementation. So I'm going to check it out and see if it suits my shop's needs. If so, I'll be happy to contribute any improvements I can make, because we would very much like this functionality to officially make it into Wagtail ASAP.

Alright! Finally got this up and running on my dev machine, and can check out how it works.

So far, I like it! That "return" icon is a good idea for indicating child relationships. And I like the way the Parent setting has a special that disables the option for setting a Collection as it's own child, or it's own descendant's child. However, I noticed a major bug. The select presented to you when you want to set an Image or a Document as a member of a Collection doesn't do the nesting. Everything's at the top level. Another minor bug is that the Root collection is shown as being at the same level as the rest of the top-level Collections, even though it's actually their parent. Was that intentional, perhaps? I'll dive into the code tomorrow and fix the nesting issue on the Image and Document forms. And also check the whole thing out in more detail, and perhaps find some other enhancements to make.

Closing this, as Nested Collections are now a thing :D Big thanks to @SebJansen, @coredumperror, @gasman and all the other devs that reviewed things along the way.