@Pesala I think we just use RSS feeds in different ways. I use RSS feeds to notify me of new posts, upon which I expect to be able to open that post in a new browser tab as easily as possible. You seem to use RSS feeds such that you want the feed reader itself to present you with a digest of the post's contents and only in certain cases will you follow the feed to its original article.
So to me, what Vivaldi does with my RSS feeds is all essentially only navigation to the feed's webpage. Third-party extensions handle this navigation in a little popup from your toolbar, Vivaldi handles this navigation in an entire browser tab. That's what I meant by the little popup taking up less screen real estate.
See this quote from my OP for what I want: "Clicking on the RSS feed should expand the latest x articles below it while staying in the sidebar, and then I should be able to just click on one and open it."
A normal RSS Feed extension will provide this behavior, and I thought the goal of Vivaldi's integrated RSS feed feature would be to provide this same behavior but as a native browser feature.