When I shutdown via terminal or by clicking the button, it just shows a hyphen on the top left corner is stuck there forever. I manually have to interrupt with the key and shut it down.
Using Dell XPS 15 9560
I have the same laptop the problem is with Nvidia driver.
you need to install "additional drivers"
sudo apt update sudo apt install ubuntu-drivers-common
Or you can open AppCenter and install "additional drivers" from there.
I'm not sure that the package is called exactly that but you can look it up for the exact name
after everything is installed open additional drivers app and select Nvidia 381 driver
that should fix your issues
I answered a similar question here. It could be a service is getting hung and can't shutdown properly. This fix adjusts the timeout for hung services.