Apparently I hit some shortcut combination that caused Slack to automatically collapse the sidebar that contains the list of Channels and Direct Messages.

There is an option to temporarily show the Sidebar in the View menu:

we now temporarily see the left sidebar

But after clicking in conversation area the sidebar will still hide itself again.

I have looked at Preferences | Sidebar and it does not have any setting related to hide/show:

Is there any way to do a “factory reset” on the Slack settings but retain the existing channels? In other words I do not want to completely reinstall Slack unless necessary.