Thanks but that’s not what I’m looking for. I have my tabs on top. What Vivaldi has is an additional sidebar that lets you display arbitrary websites inside that sidebar while the active tab from the main tab bar still takes up most of the browser window.

I find that quite useful for stuff like a feed reader, notes or a password manager. For a while I also had mastodon in the sidebar but found it too distracting. Basically every website with proper responsive design works.