See all channels inside a teams in the left panel under the Team's Name
Currently we have Teams defaulted to show you the first (up to five) channels ever created. If you want to add more channels to your left rail view, just click the ellipses next to the channel name and select “Favorite”. When you favorite the channel, it will then be “pinned” to your left rail under that team name.
Try that and let me know if that meets your needs!
I agree with Bruce and TristianG completely, but one workaround I have found in the meantime is to favorite the "1 more channel" and it will display normally. I suppose if you only have a few channels (like I do) it's an okay workaround to just favorite them all and not have to worry about it.
Bruce Chapman commented
The Channels window should not automatically disappear when making the window narrower. I prefer to have a chat client half-width on a screen. I should be able to pin the channels so I can always see them. Also, the design of the channels navigation bar wastes a lot of space with the team icon, which is just a square with initials in it. Just show the channel name and leave it to us to give them meaningful names which categorise.
The current way channels are shown is not optimal. Why do we collapse the channels if there is only one additional channel? Why not just show me that channel?
I would much rather have you show me all the channels for a team up front and let me manage what I see by collapsing or expanding individual teams.