While reading back up through a long-ish thread, I tried searching using my browser's Cmd+F find feature, and couldn't find text that I was pretty sure was there! I had to scroll up to find the text.
Presumably this is due to using some kind of lazy-rendering virtual list to avoid having a ton of elements in the DOM.
I hope searching text in a post will be possible without implementing a custom Cmd+F override - I'm always frustrated by Github's custom searching inside their editor.
I know it's a tough problem to resolve, I'm having to deal with it in one of my own projects :-x