After your recent note, I tried again to make vm window resizing work. I eventually succeeded. Here’s what I did; perhaps it will help you.
First, my environment (the host) is a system 76 wild dog pro with nvidia graphics (proprietary driver). I have just installed ubuntu mate 18.04 (64-bit). I installed virtualbox 5.2 from the repository. I downloaded a standard Ubuntu (gnome) 18.10 ISO and used virtual box to create a virtual machine for the 18.10 release. Here’s what I did that led to success.
First, I followed the instructions for installing virtualbox guest additions per the notes here. https://websiteforstudents.com/installing-virtualbox-guest-additions-on-ubuntu-18-04-beta/
I did not configure file sharing, but I did restart the virtual machine.
When the vm came back up, I noticed that when I opened the “Machine>Settings” window from the vm’s virtualbox menu bar I could then move and resize the vm window, which scaled properly. When the settings window was closed, the vm window could not be resized or moved.
Now, I don’t know if the guest additions software made the difference or if I just noticed that the vm window could be resized when the machine settings window was open.
Maybe this will help and not be too confusing.