After recent zypper dups on headless (VNC) TW installs the VNC screen turns black after sitting idle for some time. I found no way to recover other than rebooting via ssh.
I found that turning off screen blank and dim (System Settings, Power Management) prevents this behaviour on at least two installs I updated recently.
A zypper up via a remote connection can be problematic if the network is updated in the process. It is therefore better to start the zypper up in a screen session. This ensures that zypper continues to run properly even if the connection is lost. The screen session can also be reactivated afterwards.