I feel silly but after I dragged the sidebar separator, resizing the sidebar to zero width, I couldn’t see any way in the interface to make the sidebar reappear.
This is for Version 1.10.4 running on macOS.
Ultimately, I closed Everdo and manually edited
~/Library/Application Support/everdo/applicationState
and reset "sidebarWidth": 0 to 400
What’s the proper UI way of doing this?