Discussion FR: Autocomplete / list tags in post editor

It would be handy if the "tags" input box in the post editor showed a list of existing tags as you typed, since you aren't (or at least I'm not) allowed to create new tag, and it's hard to guess what exists.

There is a thing that says "available tags" except it seems to be a very incomplete list; it'll show one or two tags but I know there are many more, since I've used tags that aren't in that list (and because I can't create new tags, I know they exist already).