A demo is worth about 1,000 words:
Try to type
@ in the post body editor and you will get a mention list popup with typeahead support. I can even see a very similar feature in this very editor of Discourse that I am typing in.
I think this is a very common pattern that many PM based editor would need: Typing a special character triggers opening a popup with some type of a search support.
Is there an existing plugin that can support this?
I had a look at the Atlaskit source code for this (documentation, source code), and while it seems to be a reusable component, it has many dependencies making a minimal editor with mention support a heavy one.