Virtual Box for some reason doesn’t seem to show Ubuntu in full screen mode and the display is distorted. You will have to install virtualbox-guest-additions in order to fix this issue.
First, make sure you have the DKMS (Dynamic Kernel Module Support) package.
In a terminal window type:
sudo apt-get install dkms
Then, install the virtualbox-guest-additions by typing the following apt command in the terminal window:
sudo apt-get install virtualbox-guest-additions-iso (to search it first to see if it is available #apt-cache search virtualbox-guest-additions-iso)