[Accessibility] Improve overflowing of SubdomainNavBar on mobile

[simurai]

This came up in https://github.com/github/accessibility-audits/issues/5254. The SubdomainNavBar on mobile could run into overflow issues (overlapping) when there is not enough height and lots of items. Or in this case when zoomed in to 200%:

An option might be to add some overflow scrolling when the height runs out. But it might be tricky to fix since the extra buttons (at the bottom) are positioned absolutely? 🤔