Screen not locked when coming out of suspend, and VirtualBox is in full-window

[Shoham Peller]

To reproduce on a laptop:
1. Open a virtual machine using VirtualBox (Windows 10 machine, not sure if it matters)
2. Enter full-screen mode
3. Close the laptop's lid
4. Wait a few seconds, and open the lid

The session should be locked and you should see the lock screen. But instead you can see the virtual machine and use it.
After ~20 seconds, or after you exit the full-screen mode, the session is locked.

Maybe the virtual machine gets the ACPI signal somehow? But then why does the screen lock after a few seconds?

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: gnome-screensaver 3.6.1-7ubuntu4
ProcVersionSignature: Ubuntu 4.13.0-41.46~16.04.1-generic 4.13.16
Uname: Linux 4.13.0-41-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.17
Architecture: amd64
CurrentDesktop: Unity
Date: Tue May 15 04:02:13 2018
GnomeSessionIdleInhibited: No
GnomeSessionInhibitors: None
GsettingsGnomeSession:
 org.gnome.desktop.session session-name 'ubuntu'
 org.gnome.desktop.session idle-delay uint32 0
InstallationDate: Installed on 2017-08-01 (286 days ago)
InstallationMedia: Ubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.2)
SourcePackage: gnome-screensaver
Symptom: security
Title: Screen not locked when coming out of suspend/hibernate
UpgradeStatus: No upgrade log present (probably fresh install)