It's my one complaint about Vivaldi's implementation, yep: you can "stack" tabs, but this creates an absurdly tiny target space to click between tabs, and there's no way to create tab trees instead. It would be lovely to be able to create them, because I often create trees of tabs when browsing sites like HN: I pop tabs open behind HN as I see interesting links, and then click down the tree to go through the sites themselves.