J,
My bet is still on the new editor. Shal, is there anything in
groups.io that takes input from the user but doesn't use the editor?
If you go to the Preferences in your Account you can change your Post Preference from HTML to Plain Text. Then your Reply (or New Topic) composition box will be a plain text form control rather than an iframe from the TinyMCE editor.
Something simple like typing a string into the search box?
That, the edit box for your signature on your Subscription page, the description field when you create an event in the Calendar, the Description field when you edit a photo, a Paragraph field in a Database table. Undoubtedly some others.
Something else that _does_ use the TinyMCE editor is Wiki page creation or editing (if in HTML format). That might be a useful double-check.
Shal
Shal Farley