Actually, a little more searching found this bug, which is a duplicate - but does identify the problem as one with gnome-screensaver and discusses some work arounds and possibilities of fixing.
https://bugs.launchpad.net/ubuntu/+source/gnome-screensaver/+bug/93466 -- Do not detect Mouse activity on Second screen while on Dual Desktop Configuration https://bugs.launchpad.net/bugs/96440 You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
