For quite some time I have been hoping that somebody would provide a CodeMirror 6 integration for Parinfer. 🙂 As I never found one, I started to explore in Scittle what it would take to get it done. I got to the point where it works pretty well, but I'm not sure yet when I will find the time to take it further (i.e. turn it into a proper JS module). So, in case anybody is interested now or in the future, here's my work-in-progress: https://github.com/jurjanpaul/codemirror6-parinfer with demo editor at https://jurjanpaul.github.io/codemirror6-parinfer.
Update on the CodeMirror 6 Parinfer integration: apart from the Scittle implementation that is still maintained, this is now published as a proper CodeMirror 6 extension (ESM module written in TypeScript): https://github.com/jurjanpaul/codemirror6-parinfer (tagged 0.1.0 just now after a number of 0.0.x versions). Please note that this still doesn't do anything with the Paren Trail in the result of a parinfer invocation, mainly because it has not yet become a priority for me, but also because of my concerns w.r.t. how much state would need to be managed in the context of CodeMirror 6 to actually style them properly (particularly across undo and redo). In the spirit of compatibility I still think it would be a worthy goal to support this eventually, but I won't get around to it soon. It makes me wonder though how attached Parinfer users are, in general, to a styled (greyed out) Paren Trail. Please let me know!