[rish404]

Curious what exactly includes the "seen" feature? If I have 30 tabs opened in my browser, facebook being one of them, does that mean every message would be marked as "seen" even though I haven't visited that tab for hours? How does it work on mobile? Do push notifications also count in the "seen" since I can see and read the message on the lock screen itself.

[adventured]

I believe it means it would never get marked as seen to the user on the other end of the chat. That is, they would never know that you saw the chat text.

I assume it's interrupting or misleading whatever Facebook is using (focus? mouse movement?) to determine whether you've seen the text in the chat box.