It appears, at least for me, that on macOS there is no scrollbar for the vertical tabs feature. I really like this feature, but I find it easy to get lost when I have a lot of tabs open simply because there is no scrollbar to indicate my position. By contrast, there is a scrollbar which appears automatically when I scroll in the Tree Style Tab extension for Firefox, which provides a very similar functionality. This scrollbar helps me keep track of where I am in the tabs and keeps me from losing my place. The way it’s implemented there is ideally what I’d like to see implemented in Brave.
Currently, I don’t see any scrollbar at all when I use Brave, whether I’m using an Apple trackpad or a traditional mouse. It doesn’t seem to show up at all no matter what type of mouse I’m using, so I assume that a scrollbar for vertical tabs simply hasn’t been implemented yet, and cannot find anything about it by searching the forum or Github issue tracker.