I have created a virtual machine with a minimal CentOS 7 installation on my Ubuntu 15.10 guest system. I also installed the guest additions. During the installation the installer issued a message, that the installation of the X11 components is skipped, as I have no desktop environment installed.
However, after installing the guest additions, resizing the screen does not work, and the settings for the virtual screen resolution of VirtualBox are greyed out.
What could be the reason for this behaviour?