Screen lock may be bypassed using external display

[Ada Wah]

The screen lock in LightDM may be bypassed on a laptop using an external display, keyboard and mouse.

Steps to reproduce:
* Suspend the laptop
* Connect an external display, keyboard and mouse
* Wake up the computer by opening and closing the lid (I had to do this a couple of times) and moving the mouse about after the lid is closed
* You are now logged to the Ubuntu desktop, without entering the password

Expected behaviour:
* A password prompt should appear on the external display after the lid is closed

Please tell me if there is more info I should provide.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: lightdm 1.6.0-0ubuntu2.1
Uname: Linux 3.9.0-030900-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.9.2-0ubuntu8
Architecture: amd64
Date: Tue May 21 19:39:13 2013
EcryptfsInUse: Yes
InstallationDate: Installed on 2012-10-11 (221 days ago)
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
LightdmConfig:
 [SeatDefaults]
 user-session=ubuntu
 greeter-session=unity-greeter
MarkForUpload: True
SourcePackage: lightdm
UpgradeStatus: Upgraded to raring on 2013-04-08 (43 days ago)