secondary selection with middle mouse button not working

[Gerald Gilbert-Thorple]

On my new computer, secondary selection with the middle mouse button is not working. The cursor
does not move when I attempt to drag it while the middle button is pressed. It may be a motif bug
rather than an nedit bug, since the problem does not occur when I ssh into the problem computer
from a working system and run nedit remotely, whereas the same problem occurs if I ssh into the working system and remotely run nedit from the problem computer. But nedit is the only application where I am seeing any problem with the middle mouse button.

$ lsb_release -rd
Description: Ubuntu 20.04.2 LTS
Release: 20.04

$ apt-cache policy nedit
nedit:
  Installed: 1:5.7-2
  Candidate: 1:5.7-2
  Version table:
 *** 1:5.7-2 500
        500 http://ca.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        100 /var/lib/dpkg/status