Gnome session fails to lock automatically when VirtualBox VM is in the foreground

In Ubuntu Gnome 16.04, if a VirtualBox VM is in the foreground, Gnome will not lock the screen automatically, and will display a message that it couldn't lock the screen. This could allow an unauthorized user with physical access to gain access to the session. Locking the session manually from the menu still works.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: gnome-shell 3.18.5-0ubuntu0.3
ProcVersionSignature: Ubuntu 4.10.0-33.37~16.04.1-generic 4.10.17
Uname: Linux 4.10.0-33-generic x86_64
NonfreeKernelModules: nvidia_uvm zfs zunicode zavl zcommon znvpair nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
CurrentDesktop: GNOME
Date: Tue Sep 12 18:58:03 2017
DisplayManager: gdm3
GsettingsChanges:
 b'org.gnome.shell' b'favorite-apps' b"['google-chrome.desktop', 'evolution.desktop', 'empathy.desktop', 'rhythmbox.desktop', 'org.gnome.Photos.desktop', 'libreoffice-writer.desktop', 'org.gnome.Nautilus.desktop', 'yelp.desktop']"
 b'org.gnome.desktop.interface' b'gtk-im-module' b"'gtk-im-context-simple'"
 b'org.gnome.desktop.interface' b'clock-format' b"'12h'"
JournalErrors:
 Error: command ['journalctl', '-b', '--priority=warning', '--lines=1000'] failed with exit code 1: Hint: You are currently not seeing messages from other users and the system.
       Users in the 'systemd-journal' group can see all messages. Pass -q to
       turn off this notice.
 No journal files were opened due to insufficient permissions.
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-shell
UpgradeStatus: No upgrade log present (probably fresh install)