I'm testing it and seems to be very broken, typing things around and things jumps everywhere.
I was trying to create something like this too, because I need something that also work for mathematical writing. Let me push a version on github and update, it fixes a lot of issues.
Unfortunately it works on my own version of markdown, which is a subset of pandoc markdown, but I think one can get claude to update the parser to work for other things.
I was able to repro one issue that could have been contributing to your broken experience - there's a slight delay between, for example, clicking text in a heading and having the "#" markdown decoration appear. This is to prevent the mouse location from shifting mid-click and causing text to be selected unintentionally (obsidian does this too). But there was a bug that was causing a cascading set of failures if edits happened during that delay window, which is likely what folks who are clicking around at random points in the editor and adding text are doing. I fixed it in 0.4.2, which should be live now.
Thanks for trying it out! would you mind giving some steps that allow me to repro the issue? It's early days so i'm sure there are some rough edges, hopefully I can fix them quickly.
My version of a WYSIWYG built on top of CM6.
https://github.com/chaoxu/coflat
Mine also have lot of bugs (especially reader and editor doesn't completely match yet).
repro the issue: click random places and add random texts, scroll around, and issues come up sooner or later.
I just clicked around a bit and randomly typed, suddenly no matter where I typed, my text was ending up at the end of the document. Found it to be pretty broken as well.