Decided to persevere with Vivaldi, not sure I like Firefox 😞
So I made a list of all the bugs I've been encountering recently and will attempt to log everything. I've had so much trouble with Vivaldi yet it is still the best browser in terms of customizability and features, but it's such a sad state to be in that I have to choose the least worst browser in this day and age. I've just lost a whole stack of tabs all of a sudden, and some of the recent history is also gone - this has got to be the worst bug I've encountered yet, don't even know if there is any point in logging it.
Anyways regarding this stack bar - which was still a nuisance on the latest version from Google Play - I went through all the settings and experimented with stuff and all of a sudden, the issue is GONE. So I can see why it can't be reproduced and I can't see how it can be 'fixed' without proper reproduction steps. This seems to be somehow related to invalid settings, so if anyone still experiences this issue, play with your settings, in particular, put the address bar at the bottom, then back to the top.
It's still a little annoying that when Vivaldi starts, the stack bar still appears briefly and then disappears when it's not applicable.