Well, so far no luck. I tried both Gnome 3 and Cinnamon and the problem persists on all three DEs on CentOS 7. So, I don’t know what to think now. Everything else in UE4Editor seems to work fine on CentOS 7…except that I had to open port 6666 (UDP) to get Lightmass to work, whereas I didn’t have to do that with Kubuntu. Once I made the firewall change on CentOS, Lightmass worked great.
Very puzzling, this marquee selection thing. Wonder if there is a setting in CentOS that affects that behavior that I don’t know about. Seems kind of like a long shot, but I’ll poke around.