Selected categories should always be at top

I was looking at this ticket and I noticed a side effect of moving newly added categories to the top of the list, namely that they get moved out from under sub-categories.

I had:

- AAA
- Asia
  - Anime
  - Pho
...

I added "Pho" as a category to the post, and the list changed to:

- Pho
- AAA
- Asia
  - Anime

I'd expected that Pho would simply be moved to the top of the Asia category, and maybe Asia moved above AAA? This feels immensely more complicated, of course, but is it right to appear to "re-parent" categories like that?

There's another issue... what if "Pho" has children? Do they move up to the top with "Pho" ? Do we keep another copy of "Pho" down where it should be in the tree?

It sounds hard to do this way.

I'm leaning towards a more Flickr-like system.

example:

Assigned categories:

[x] PHP [x] Technology >> Blogs [x] Geology

Assign additionally categories:

[ (text input box) ] [Add >>]

[Category tree]

Note how Technology >> Blogs shows the parents of "Blogs," so you represent the tree structure instead of a flat tag system like with Flickr. Clicking the [x] next to an assigned category would unassign it (asynchronously).

If JS is available, the text input box would add asynchronously, and the category tree would also add asynchronously. The category tree should also remove categories asynchronously when the checkbox is unticked (it would remove it from the list above, as well).

They could work in concert, or, people could hide one portion or the other, making it like a pure tag system or a pure category system.

I should probably do a mockup.

Moving my idea to #3130

No traction in over a year, so closing as wontfix.

Feel free to re-open if you have additional information/patches/suggestions/...