@Viqsi I'm pretty sure Vivaldi's webapp UI is unique in the browsing universe. It is an additional layer and thereby an expanded area where not only Vivaldi code changes, but also Chromium changes can trigger errors seen nowhere else. Further, every single machine that runs Vivaldi creates a unique environment, non-replicable by devs or testers, where some incompatibility or error may arise.
One way Vivaldi tries to defend against unanticipated friction in the interface, is to have (around seventy at present, I think) volunteer internal testers (called Sopranos), using Vivaldi on every age, capability and configuration of hardware possible, as many hours a day a possible, report back on every cock-up detected. So if any of my machines trips over anything, I report to devs ASAP. Same with the other Sopranos.
I not only use the bleeding edge of Vivaldi as my daily driver some 17 hours a day on my main Windows machine, I also test on three more towers (both Windows and Linux on one of these) and an old X86 laptop here in my office, two newer laptops that don't get turned on every day, a phone, a tablet, and two more towers (one somewhat obsolete) at my remote office when I'm there. To make matters worse, I read all the English posts in the Forum daily and try to reproduce bugs reported by users.
I'm not sure other Sopranos test on as many different kinds of hardware - in fact I'm sure they don't. No one hordes computers like me.
But when the devs and testers cannot reproduce an issue, there is no way to address it. Luckily if, say, even ten percent of users have a specific issue caused by a specific bit of code, one or more of the testers or devs is going to be able to reproduce it, and it's going to get tracked down. It's for this reason that thirty to fifty issues are resolved by the team every. single. day. And I don't even recognize most of the issues in the daily changelog, so I know even obscure glitches are being detected and remedied all the time. Yay team.