Vlad You know what, you're right, I haven't seen this in any Apple app to date. Force touch behavior comes to mind, but it's different. I've come to think about it - hover is not very good for accessibility, probably that's why it's not been widely used. Though I would still love for the tab bar to become narrower at it's min width, alike to the example I've sent you. Perhaps detaching a tab bar from the "traffic lights" buttons would be a good call here (idk what's the alternative option as per guidelines though).
What about a manual expand to become easier by the way? Maybe we could do something like an arrow button to expand the vertical bar (a hotkey would be desperately needed, too). Usually there's a button to hide the side menu (Think "Console" app, but a lot of Apple apps have that pattern), but a small preview is still essential for a complete browsing experience, so a button nearby the Window switcher (or maybe inside the tab bar itself) to expand/collapse it without manually fiddling with it would be great. As an addition you could also implement "magnetizing" to a minimum and default value, so that it would be easier for user to set up the tab bar width.