Hmm … seeing screenshots of the Java app and writing about the embedded HTTP server trigger thoughts of my local installation being significantly outdated. (The Java app shows version 5.2, so I guess I’m missing all of the 2021 releases. My bad. I’ll update.)
Still, it’s remarkable that the feature does work with at least one page (opening post, first shot).
Postscript
Now I realise, from testing in a different browser without the extension:
- what’s in the first shot does not require the extension
– it’s simply a demo page.
So, my problem is almost certainly local, I expect to resolve it with an update to the local server.
Apologies for the noise. tl;dr it was the demo page that threw me.