Comment 57 for bug 1684240

I began having this problem last fall after an update to Nvida from 378 to 384, IIRC.
A couple of weeks ago I installed Kubuntu 18.04 with the Nvidia 390 driver and did NOT have this problem.
I switched back to Neon for personal preference reasons.
A workaround that I've been using is to open a console on the login screen (which shows the mouse pointer defect) using Ctl+Alt+F2 and in that console enter:

sudo systemctl restart sddm

The causes the login screen to reappear and the mouse defect is gone. I continue the log in and the Plasma 5.12.5 desktop appears and behaves normally. Of course logging into the desktop, logging out and logging back in also works but is too slow. The next time I boot up and the mouse artifact appears on the login screen I open the console and use the up arrow to locate the command and hit the Enter key on it.