Fix 'Activity' message status
Let's say you're available on the channel and your teammate is writing more than one message. You look at 'Activity' icon and see that counter displays 4 unread messages. You want to read them all in some order, but suddenly you see that all your messages are already read. For me there should be 'mark all as read' option, as in MS Outlook.
Now all messages become read with one click automatically, which is misleading for some teammates of mine.