Ressurection of #23549. Fix regression https://github.com/go-gitea/gitea/pull/23513#issuecomment-1474356817 from #23271. The previous sticky CSS did assume the content is always 2 rows, but since that PR, it's single-row above 993px width. Adjust the sticky offset to match and add a small tweak that hides content behind the `border-radius`. Single row: <img width="1264" alt="Screenshot 2023-03-17 at 21 33 05" src="https://user-images.githubusercontent.com/115237/226034050-a04b131d-fd3f-45c0-bc72-413738a59825.png"> Double row: <img width="1243" alt="Screenshot 2023-03-17 at 21 32 53" src="https://user-images.githubusercontent.com/115237/226034163-2f1c6aa9-fc72-432f-bc46-9a7119da8677.png">
