To summarize the problem:
The default Vivaldi behavior is . . . when you keep opening tabs, the width of the tabs eventually becomes so small that the tab-closing button occupies most of the tab area. Then, when trying to activate a tab, you accidentally close it and are annoyed.
In the feature request forum, there have been two requests that, if implemented, would solve the problem in one way or another.
Add the ability to set the minimum tab width (whether the tab is active or inactive). This request, from 2018, has gained some support but haven't been implemented.
Show the tab-closing button only when the tab is active. This request didn't get many upvotes and have already been archived.
I mean to propose a variation on request 2 above.
Hide the tab-closing button when the tab is inactive and too narrow.
But, looking at the fate of request 2, I'm discouraged. Also, I think that request 1 is a better solution.
[Aside] The original problem I described at the top, is a real problem. Instead of, or in addition to, offering some options to fix it, Vivaldi wants to change its default behavior.
Not only about this issue, but in general, I imagine there are a lot of newcomers who try Vivaldi out, is annoyed by some of its default behaviors, and quietly ditch it.
Only the minority of those users who are frustrated will post here in the user forum. So, if the default behavior is suboptimal, offering an option or workaround isn't good enough.