What does HackerNews think of tui.editor?

🍞📝 Markdown WYSIWYG Editor. GFM Standard + Chart & UML Extensible.

Language: TypeScript

#6 in Front end
#10 in HTML
#20 in Markdown
https://github.com/nhn/tui.editor Might be close to what you are after. The only thing I don't think it does is the @mentions, but there is probably a way to address that.
it is based on an open source markdown editor https://github.com/nhn/tui.editor

About the codemirror css, it is there because this editor uses it to highlight markdown code