> The problem wasn’t that the line I wanted wasn’t on the page—it’s that the whole document wasn’t being rendered at once, so my browser’s builtin search bar just couldn’t find it.

Ctrl/Cmd-F is intercepted on blame/code pages and opens a custom find UI, which honestly seems fine to me. Browser find-in-page isn't meant for code search and it's pretty slow.

Maybe GitHub is switching to React (I don't know anything about that), but React doesn't mandate, or even natively support, culling offscreen content, so I'm not sure what the connection is there.