Scotty-Trees could be worth a try when it lands in Solus
Thanks for pointing that out. Having had a career developing commercial software, I understand that expectations need to be lower for any beta version of a product than for a release version. And as I've mentioned, Thunderbird has served me well for more than 20 years, so it's not as if I actually need these new features in Vivaldi right now. So I can be very patient and wait to see them working at some time in the future.
It's just because Vivaldi has announced that these features are here now, and their documentation describes how to enable them and how to access them (none of which worked here), and comments in the forum led me to believe that others had succeeded in getting them to work, that my expectations were perhaps a bit higher than they should have been.