Hey HN, I'm one of the founders of Nextjournal. The `cm6=1` query param is a feature flag to enable our new Clojure code editor built on CodeMirror 6 [1] with lezer for fast incremental parsing. You can find the keybindings on [2], be sure to try grow / shrink selection.
[1] https://github.com/nextjournal/codemirror.next-clojure
[2] https://nextjournal.github.io/codemirror.next-clojure