Ubuntu 20.04 Desktop glitch [Small green box over the screen]

Solution 1:

To temporarily fix it pressalt+f2 then r.

To actually fix the issue, run these commands in the terminal.

sudo apt remove --purge xorg xserver-xorg gnome-session ubuntu-desktop-minimal ubuntu-desktop gdm3 xwayland
sudo autoremove --purge
sudo apt install gnome-session ubuntu-desktop-minimal
reboot

Read this for more information.

Solution 2:

I just deleted /etc/X11/xorg.conf.d and its contents (20-intel.conf), then rebooted, and that seems to have done the trick. I don't completely understand but it seems having that file (20-intel.conf) present, causes an old Intel package/driver (xserver-xorg-video-intel) to be loaded, instead of using the (recommended?) modesetting driver; check here for more info. After rebooting, you can check what changed by comparing Xorg.0.log and Xorg.0.log.old in ~/.local/share/xorg (or wherever your Xorg logs are located). You should notice new entries with modeset(0): instead of intel(0):.

Now, I originally created that conf file to solve tearing issues on 19.04 (which got solved then but still had the occasional coloured glitches on Firefox). Hopefully, they won't return.

PS: Worth mentioning that before deleting said folder, I'd already tried the package purge and reinstall, recommended in the other answer, which didn't work. So if deleting the Xorg conf doesn't work for you, you may also want to try that purge and reinstall as well.