I had a 16.04 version of Ubuntu running in virtualBox (Host OS is Windows 10 Enterprise). I just upgraded to 18.04. Also updated the kernel to 5.x fom 4.x. 16.04 had VBox guest additions installed, so a bidirectional cut and paste from host to guest was working, shared folders were working, guest OS (Ubuntu) display would occupy the whole screen available witin virtual box. After the upgrade the cut and paste still works, shared folder works, but guest OS display does not occupy all the space in the virtual box, there is a gap all around the window.
I have tried reintalling VBox Guest Additions but has not helped. Any ideas on what else I need to do. Almost all articles I read suggest installing GuestAdditions which I have done and redone.
After the OS upgrade I had to add to edit /etc/default/grub to add 'nomodeset' in the below line. Apart from that I have done no changes. I had to do this as the splash screen would freeze with the ubuntu logo.
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nomodeset"
EDIT
I just noticed that in the VM with upgraded OS I have 'Auto Resize Guest-Display' under view Disabled. Also in View->'View Virtual Screen1', the Resize 800x600 is selected. Neither of this happened when I had 16.04 installed (I compared with another 16.04 that I have running).
In File->Preferences->Display, I have tried having Automatic selected (this was the 16.04 setting). I then changed Automatic to Hint with a recolution of 1920x1200 (This was suggested in other answers in this forum)