But they do have constant height in the sense that, unless you resize the window horizontally, the height doesn’t change.

For what it’s worth, modern browsers can render absurdly large plain HTML+CSS documents fairly well except perhaps for a slow initial load as long as the contents are boring enough. Chat messages are pretty boring.

I have a diagnostic webpage that is a few million lines long. I could get fancy and optimize it, but it more or less just works, even on mobile.

Dynamic height of virtual scrolling elements is a thing. You just need to recalculate the scrollable height on the fly. tanstack's does it, as do some of the nicer grid libraries.

I'm ignorant of the tech here. But I have noticed that ctrl-F search doesn't work for me on these longer chats. Which is what made me think they were doing something like virtual scrolling. I can't understand how the UI can get so slow if a bunch of the page is being swapped out.

They didn't actually name the solution: the solution is virtualization.

They described Netflix's implementation, but if someone actually wanted to follow up on this (even for their own personal interest), Dynamic HTML would not get you there, while virtualization would across all the places it's used: mobile, desktop, web, etc.