lock screen disabled by virt-viewer

[Tedesco]

When you lock the screen by mouse click on the 'lock icon' you can experience the pop up of the message:
"No se pudo bloquear
Una aplicación impidió el bloqueo"
=> To lock wasn't possible
An application disabled the blockage

On the photo attached you see this situation.

To reproduce this phenomenon is complicate, it depends on the number of windows and the position of the left window on the screen area. And perhaps the focus of the first window.
In this case these two windows are virt - viewer applications.
With windows of the Firefox it wasn't possible to produce the situation.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xorg 1:7.7+19ubuntu7.1
ProcVersionSignature: Ubuntu 4.15.0-39.42-generic 4.15.18
Uname: Linux 4.15.0-39-generic x86_64
.tmp.unity_support_test.0:

ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CompositorRunning: None
CurrentDesktop: ubuntu:GNOME
Date: Tue Nov 20 11:17:36 2018
DistUpgraded: Fresh install
DistroCodename: bionic
DistroVariant: ubuntu
ExtraDebuggingInterest: Yes
GraphicsCard:
 Intel Corporation 82G33/G31 Express Integrated Graphics Controller [8086:29c2] (rev 0a) (prog-if 00 [VGA controller])
   Subsystem: Dell 82G33/G31 Express Integrated Graphics Controller [1028:0294]
   Subsystem: Dell 82G33/G31 Express Integrated Graphics Controller [1028:0294]
InstallationDate: Installed on 2016-03-17 (978 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
MachineType: Dell Inc. OptiPlex 360
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-39-generic root=UUID=8a9d54df-bf77-4277-990d-781804a2250b ro quiet splash vt.handoff=1
Renderer: Software
SourcePackage: xorg
Symptom: display
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 07/02/2009
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A02
dmi.board.name: 0T656F
dmi.board.vendor: Dell Inc.
dmi.board.version: A02
dmi.chassis.type: 3
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA02:bd07/02/2009:svnDellInc.:pnOptiPlex360:pvr:rvnDellInc.:rn0T656F:rvrA02:cvnDellInc.:ct3:cvr:
dmi.product.name: OptiPlex 360
dmi.sys.vendor: Dell Inc.
version.compiz: compiz 1:0.9.13.1+18.04.20180302-0ubuntu1
version.libdrm2: libdrm2 2.4.91-2
version.libgl1-mesa-dri: libgl1-mesa-dri 18.0.5-0ubuntu0~18.04.1
version.libgl1-mesa-glx: libgl1-mesa-glx 18.0.5-0ubuntu0~18.04.1
version.xserver-xorg-core: xserver-xorg-core 2:1.19.6-1ubuntu4.2
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev N/A
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:18.0.1-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20171229-1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.15-2
xserver.bootTime: Wed Nov 14 13:05:37 2018
xserver.configfile: default
xserver.devices:
 input Power Button KEYBOARD, id 6
 input Power Button KEYBOARD, id 7
 input Dell Dell USB Keyboard Hub KEYBOARD, id 8
 input Dell Dell USB Keyboard Hub KEYBOARD, id 9
 input Microsoft Basic Optical Mouse MOUSE, id 10
xserver.logfile: /var/log/Xorg.0.log
xserver.outputs:

xserver.version: 2:1.19.6-1ubuntu4.2