Comment 80 for bug 49579

It is still present in Ubuntu 16.04.

All as described here: https://bugs.launchpad.net/ubuntu/+source/gnome-screensaver/+bug/1124282
"If the Windows window is on focus and i am inactive, then my screen is locked after a certain amount of time and should normally ask for my password. But it only does so if a change the focus to a window other than my virtual machine."
but it will be locked after a certain amount of time then and only then you click on some area of the host (it's not enough just to move the cursor outside of vm). Otherwise, you will be able to work inside the vm.

Moreover, in this case (in conjuction with VirtualBox 5.1.28 r117968) it is possible to bypass the lock screen and unlock the host without needing to enter a password. All you need is just double click inside the vm window (lock screen will blink and disappear). After that you will be able to interact with the host.

Steps to reproduce:
1. System Settings > Brightness and Lock.
2. Set "Turn screen off when inactive for" to 1 minute.
3. Turn on "Lock", set "Lock screen after" to "Screen turns off", check "Require my password when waking from suspend" checkbox.
3. Focus to the virtual machine window.
4. Wait 61 seconds.
5. Double click inside the vm.