Some Widgets/Plugins will not work correctly with variants. A simple workaround is to publish the widget in a separate document without any variants using, for example, a subdomain (e.g.
https://myforum.mydomain) with (optionally) scale-to-fit-width; you can then call 'myforum' from your primary website (e.g.
https://mydomain) for each variant; standard menu buttons etc. You can style both sites to match (page size, colours etc.).
The approach appears to work well for some widgets. See previous post:
https://www.talkgraphics.com/showthr...hare-it-as-one
The upside would be you are managing your forum just once and likely it will scale correctly across multiple devices. You can choose the make 'myforum' a supersite too, which gives you animation options.
Cannot tell if the approach would be successful with the Nabble forum, but worth a try.
Gary