I've also discovered that Vivaldi's built-in password manager will simply not detect a login page in various cases, and thus never provides a password prompt, even on encypted pages.
I'm personally not a fan of this kind of "nannying" behaviour wrt refusing to work on http only pages, because I also run a variety of devices on internal networks which have no need for encryption. (and which may be older hardware that actually has SSL/TLS support, but if you actually enable it the UI becomes hopelessly sluggish)
But we are all being dumbed-down to the lowest-common-denominator of web users these days, obviously.