Screen flicker on wake-up

[chronosJ]

I know this one very well. @DotWayton, the gear thing is on the right track, but doesn't stop Wayland on the login screen and the flickering would continue unless you totally disable Wayland:

@chronosJ

had similar things happen when I was using Wayland, but it wasn't enough for me to just change the gear icon to X11/Xorg (I think the login screen still used Wayland, which still caused flickering -- sometimes until I rebooted!)

So, edit a config file like so:
sudo nano /etc/gdm3/custom.conf

Then, uncomment [remove the # from] the line containing:
WaylandEnable=false

Save it and reboot.