Markdown Editors
Awesome list of markdown editors but not all of them provide visual in place editing - https://github.com/xjh22222228/awesome-web-editor/blob/main/README.md#markdown-editor
I'd point out some browser markdown editors but they don't have mdx support afaik:
Milkdown
https://github.com/Milkdown/milkdown
- ⭐ 10.5k (2025-09-01)
- Status: 🏃♂️ (active)