@pesala I changed the position of tab bar and finally see your point. I originally responded to how the browser work when you use default settings (non-native look of the window + tab bar on top). It is a question where the bug is since we do not know what intention Vivaldi developers have. Since impossibility to use middle click to open a new tab when having the tab bar on top lasts for many months already, it rather seems to me like a (IMO bad**) design decision rather than bug (which would be easy to fix many months ago, especially when it is present in default settings and is pretty obvious).
** it is a sin to not utilize the empty space on the tab bar
My linked post still remains relevant, since it describes current state in more detail and illustrates a simple way of how to inform a user that the whole tab bar area is "active" for various gestures or middle click. The new "+" icon would not steal place for titles of pages inside the tabs, since the icon would shrink as you fill the tab bar with tabs.