gdm reveals the last screen right before locking screen after screen turned black

[Karl-Philipp Richter]

After my screen is turned off due to inactivity and I return to the PC (after the screen has been turned off for some time) and move the mouse the login screen (for locked session) is displayed, but not immediately - the last visible screen before I left the PC is visible for some instants (can be up to several seconds if the system is under load) and reveals what should be protected by session locking. `lightdm` does the job better by showing the session lock screen immediately.

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: gdm 3.10.0.1-0ubuntu7.1
Uname: Linux 3.19.1-031901-generic x86_64
ApportVersion: 2.14.7-0ubuntu8.2
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Mar 13 18:18:42 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-01-26 (45 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: gdm
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.