VirtualBox 6.0 was released on Dec 18th, 2018. This is a major version upgrade. The biggest improvement for me is the HiDPI support. It makes the Ubuntu VM in my MacBook host be able to leverage the high resolution display.
First you need to install the guest additions from “Guest Additions CD image…” as before.
After rebooting, the icons and fonts are really tiny since it’s under a very high resolution rate. Go to “System -> Displays”, change “Scale” from “100%” to “200%” and confirm. Now you will find the icons and fonts are big enough and the edge of them are much sharper than before.
One issue is that, if I change the VM window size, the “scale” setting will be reset to “100%”. Fortunately we don’t need to resize the window very often, but I hope this issue will be fixed soon.