How to stop display from turning off when I disconnect a Screen Sharing client
Run this command in the terminal to fix the external screen going blank on VNC disconnect. Works with Sierra OS.

sudo defaults write /Library/Preferences/com.apple.RemoteManagement RestoreMachineState -bool NO




More context from jabenninghoff on GitHub:


  In newer versions of macOS, screen sharing will automatically re-lock the screen when you disconnect if it was locked when you first connected.