Apologies: I never saw this response asking for more information.
I did solve the problem, as it turns out; it was Gromit-MPX, a tool I
use allow me to draw on the screen. The problem disappears when I
restarted Gromit-MPX.
--
You received this bug notification because you are a member of Ubuntu
Public bug reported:
I'm using Ubuntu 20.04.1, on a Lenovo Thinkpad T14s. I've got fractional
scaling enabled (125%) with Gnome.
I regularly plug into an external monitor with an HDMI connection, and
run both screens (the external monitor and the built-in laptop one)
side-by-side. With that setup
