This is a very old bug with the bottom border-radiuses not being there
and the `:has` selector now makes it possible to cleanly solve it. It
affects all header+segment boxes, which there are many throughout the
UI:
<img width="1017" alt="Screenshot 2024-04-23 at 20 47 21"
src="https://github.com/go-gitea/gitea/assets/115237/870fe352-cc38-4bd6-bfe6-9fe8c3066f92">
(cherry picked from commit 3f19a6334575e1d2849999e8339f1b515cefaf1a)
