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I had same problem. I reinstall xorg software and it fix the problem with shadows in gazebo.

    sudo apt-get install xserver-xorg-core

remove xorg-core hwe 18 automatically, but you can continue using your desktop environment. And later

    sudo apt-get install xserver-xorg-core-hwe-18.04 
    sudo apt-get install xserver-xorg-input-all-hwe-18.04

I don't know why it's worked for me. Proofed in Ubuntu 18.04, gazebo 11.