I use a third party mobile app and will scroll a thread. I'll either add a stand-alone comment after my first run through, or I will reply to a comment that's already been made. Then I close the app. I only open it again if I get notified not something in my inbox. I don't go and read through the entire thread again. I just read the reply and either respond or close the app if I have nothing to add. The app only shows the comment you're replying to. So again, I would have never seen the reply unless I wanted to scroll through the thread again. Which it was over 100+ comments at that point so I had no interest in reading the same comments over and over again in the hopes that I'd see something new.