As per the code above, the UI is to be shown when the beginRequestWithExtensionContext() method is called, inside the web extension. I am referring to a native MacOS UI, I'm not referring to HTML or any browser based mechanism.

Alas this didn't fix it for me. In my case the breakpoint worked fine in November 2021, but has now stopped working in February 2022. No change to the code, or to the project.