When Vertical Tabs are collapsed, you cannot see that a new tab was opened because the tab sidebar is hidden and it opens on mouse hover (user preference).
With normal horizontal tabs, a new opened tab is immediately visible in the tab bar. With collapsed Vertical Tabs, that feedback is missing.
Opening a new tab should give a tiny visual signal so the user knows a new tab was created.
The idea is basically:
New tab → favicon briefly appears from hidden sidebar → blink → disappears.
something like that.
