Oracle VM VirtualBox uses special window manager facilities to switch a multi-screen machine to full-screen on a multi-monitor host system. However, not all window managers provide these facilities correctly. Oracle VM VirtualBox can be configured to use a legacy method of switching to full-screen mode instead, by using the command:

VBoxManage setextradata global GUI/Fullscreen/LegacyMode true

You can go back to the default method by using the following command:

VBoxManage setextradata global GUI/Fullscreen/LegacyMode

This is a global setting.