When chatting with a person on RHS and a different channel is open in center, the messages read on RHS is still marked as unread, until the DM/GM is opened in center channel.
1. Start a DM with a different user.
2. Reply to the message to start a RHS thread.
3. With the message thread still open in RHS, switch center channel to a different channel.
4. Continue message thread in RHS.
Expected: When a new message comes in in RHS, since there's an active conversation, the last message from the other user is not marked as `unread`.
Observed: When a new message comes in in RHS, even though there is an active conversation, the last message from the other user is marked as unread, and not cleared from the unread list until is the DM is opened in center channel