Not being able to figure this out, I reinstalled Sketchup. I trashed the previous Sketchup app folder, and the folder in Application Support (Mac), but may have missed others relevant system files.
With a fresh installation, the very same behavior occurs. No plugins other native SU ones.
I brought in a 2nd section plane. It is selectable. The original section is now selectable too.
Quit the program and reopen - both now cannot be selected other than crossing window.
I found this old post
but, resetting workspace did not change anything.