Marijn, the ProseMirror author, does an excellent job building fully responsive frameworks. Out of the ContentEditable libraries I’ve tested, ProseMirror did the best on Android, which is the most challenging platform to support. His CodeMirror is by far the best code editor for mobile too. Monaco, the VS Code editor component, is pretty broken on mobile.
I was just about to write the same thing. Adding here, the headless approach allows you to optimize your editor for mobile pages from a UI perspective.
Note: https://github.com/yjs/yjs for collaborative "document edition, and user cursors"; has WebRTC, web socket, matrix.org backend