I hate that content is rendered so inefficiently today that the whole document can't be rendered at once and breaks the browser builtin search. It feels like such a regression in modern software.
Slack does this and I absolutely hate it. It's impossible to search for a keyword in a long thread. The native app will simply hijack the search function, and even opening it on a browser won't work because it only renders the part that's on the screen. I'm sure someone felt really clever when they implemented it.
It really is worth considering the web an OS or an app platform these days, given that so many sites are web apps rather than web pages.
If only there was an established tool for scrolling up and down a document that didn't fit on screen