mariasx I'm currently working on updates to the Files modal popup, and I might therefore be able to implement a markdown editor while I'm at it. As I'm sure you will understand, I don't intend to write my own markdown editor from scratch. Therefore, we are limited to the available Javascript markdown plugins available, some of which may be too complicated, and some which have missing features. I would have preferred to keep it as simple as possible, with just an input editor with side-by-side preview, but I don't know if that even exists.
This is pretty much what I can find:
Cherry Markdown Github
Looks great, but very complex, very heavy and docs in Chinese.
https://tencent.github.io/cherry-markdown/examples/index.html
Tui Editor
Complicated, but might be an option.
https://ui.toast.com/tui-editor
TinyMDE
Simple, but doesn't seem to have preview mode or side-by-side view.
https://jefago.github.io/tiny-markdown-editor/
easy-markdown-editor Github
Not too complicated, and has side-by-side view with scroll synch. Might be the best option.
https://stackblitz.com/edit/easymde?file=index.html
Showdown website
Looks pretty good, but doesn't have scroll synch.
http://demo.showdownjs.com/