How to force Ubuntu to detect external display?

My laptop recognises external display devices if I connect them before booting the system, but many times it doesn't if I connect while the system is running. In this case, rebooting naturally solves the problem.

Is there a way to force Ubuntu to detect external displays? Opening up the display menu and pressing "Detect displays" does nothing.

Actually, you don't need to log out. Simply going to a VC with ctrl-alt-F1, restarting x with sudo service sddm restart and and going back to your graphical interface with ctrl-alt-F7 (or F2) should do it.

This way, you don't lose all your windows...