Not sure how to achieve this with the 'tablist' role, which is already there. We don't use the 'tabpanel' role, but that seems unrelated. On the one example I found using the 'tablist' and 'tabpanel' roles, it doesn't treat the tablist as a container. Maybe this is a VO / WebKit bug? In other macOS apps like Safari and Xcode, the entire tab bar is either one container (Xcode), or a container plus another control for the new tab button (Safari).
<rdar://problem/28235045>