What does HackerNews think of tui.editor?
🍞📝 Markdown WYSIWYG Editor. GFM Standard + Chart & UML Extensible.
Language:
TypeScript
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