I don’t see that pen listed in Derk Jan Speelman’s pens. Most, if not all, of his public pens are from 2017 and use an older version of CodeMirror. This pen of Derk’s seems to do what you are looking for though. Version 6 has just been released and has some breaking changes but you can follow Marijn’s Announcement or his Migration Guide to get similar results with the new version.