cat << EOF | sudo tee /etc/gdm/PostLogin/Default
#!/bin/bash

auth=\$HOME/.Xauthority
if [ -e \$auth ]; then
        diff <(xauth extract - \$DISPLAY) \\
        <(XAUTHORITY=\$auth xauth extract - \$DISPLAY) > /dev/null \\
        && exit 0
fi
xauth extract - \$DISPLAY | XAUTHORITY=\$auth xauth merge -
EOF
sudo chmod +x /etc/gdm/PostLogin/Default