X is misrendering a rectangle around my mouse pointer

I found comment on bug report and it helped me (KDE NEON + nvidia driver)

Crippling your system (switching to xrender and much slower nouveau driver) is not a solution.
What works for me flawlessly for more than a week on KDE Neon:
In file:
/lib/systemd/system/sddm.service
Add:
ExecStartPre=/bin/sleep 10
After [Service]
The problem apparently is that something SDDM needs is not initialized before is starts, so delaying SDDM by 10 sec is adequate workaround. Perhaps it could be less, but it doesn't bother me as long as I don't have to log out/log in manually.