I can reproduce the problem. Somehow, there is an issue when you delete all default channels, disable public channels, and there are no DMs. Everything works fine until you open full-screen chat. That does not work, and because the chat button opens the chat in the mode you used last, the button is broken.
As a workaround, you can change one of the parameters needed to reproduce the issue, e.g., start a DM or enable public channels.
2 Likes