A few days ago I visited a website, in Chromium, that had a graphic on it. After leaving the website, that graphic was superimposed over my Thunderbird email account (see image). Mousing over the artifact makes it blink in and out, and moving the Thunderbird window moves the graphic along with it. What's worse, it persists across reboots.

Any ideas how to get rid of this artifact?

Could you be a tad more detailed about your Plasma version, distribution and version? I also move this to the correct subforum, as this is not about a graphical application, but the graphics system of the desktop

Thanks for the move, I saw other posts in this forum about graphics issues, but I see they've been moved as well. Here's some more info:

Operating System: KDE neon 5.14
KDE Plasma Version: 5.14.0
Qt Version: 5.11.1
KDE Frameworks Version: 5.50.0
Kernel Version: 4.15.0-36-generic
OS Type: 64-bit
Processors: 8 × AMD FX(tm)-8350 Eight-Core Processor
Memory: 7.6 GiB of RAM

NVIDIA chip and driver, OpenGL version 4.6.0 NVIDIA 396.54.

Updated the NVidia drivers to 410.57 and the issue is still persisting.