I have upgraded from Ubuntu 19.10 to 20.04, and now I have a problem with my graphics rendering (strange squares on background and font problems). I use the i915 driver. Graphics card: Skylake GT2 HD Graphics 520. I tried to reinstall GNOME shell and Xorg server. Nothing helped. Also Chrome browser breaks on most of pages.
I had the similar problem and on my research found that it's intel new graphic update problem. I was able to solve my issue by issuing following commands on my terminal.
sudo apt purge xserver-xorg-video-intel
After issuing the command, it is recommended that you restart your pc but logout and log back in may also work fine. I hope this will be able to solve your problem. If you face any issue please let me know.