Opening a conversation used to mark it read even when the agent never scrolled to the new messages or the tab was in the background. Reply notifications in the bell also stayed unread after the agent read the reply.
Now the browser tells the server which message the agent saw. It only does this when the end of the thread is on screen and the tab has focus. The server moves the agent's last seen time forward to that message and never back.
A reply notification counts as read when the agent's last seen time is at or after the reply. "Mark as unread" marks the old reply notifications read first, so the bell stays the same.
Reply emails and pushes wait a little before sending. They are skipped if the agent already read the message or can no longer see the conversation.
Abhinav Raut