There is a little bad UI. Unread messages in another channel are placed where user cannot read easily. When user switch channel, user should scroll to most recent message to see.

It will be a better way that scroll to end of message with difference background color to show separation of read/unread messages, when user switch a channel. After some time (-30sec) passed or start typing a message, it reset background color of unread messages..

User can see most recent messages in another channel without scrolling and recognize immediately with this improvement.