Why is the Ubuntu VM screen corrupted on my HiDpi laptop?

On my macOS host, increasing the amount of processors allocated to the VM from 1 -> 4 fixed this issue for me. My guest was Lubuntu 18.04.

Basically, the allotted default amount of VRAM (video ram) in Virtualbox is not sufficient for the high-resolution display. Simply try increasing it. By increasing it from the default (19MB?) to 128MB the problem went away.

This might also be related to the Unity desktop environment, which according to the specs requires a video card of at least 256 MB, a requirement most integrated solutions will fail. In that case, try another desktop environment, such as XFCE (XUbuntu) which does not have a 3D rendered desktop by default.