More investigation shows that this is actually a Unity bug. The black screen you are seeing is the lock screen but Unity doesn't have a lock screen when in guest mode. The lock screen is triggered when you switch users via the indicator.
You can also trigger this by typing alt+ctrl+L inside the guest session. It will show a black screen that disappears when you move the mouse. The solution should be that Unity does nothing when the lock screen is requested to be activated. ** No longer affects: lightdm ** Package changed: lightdm (Ubuntu) => unity (Ubuntu) -- You received this bug notification because you are a member of DX Packages, which is subscribed to unity in Ubuntu. Matching subscriptions: dx-packages https://bugs.launchpad.net/bugs/1405349 Title: switching to activated guest session causes screen blank Status in OEM Priority Project: Confirmed Status in OEM Priority Project trusty series: Confirmed Status in unity package in Ubuntu: Confirmed Status in unity source package in Trusty: New Bug description: Hi, I switch to activated guest session, but the screen remains blank. I have to wake it up with keyboard/mouse action. How to reproduce: 1. login with your account normally. 2. switch to guest session from the panel menu "Switch Account... -> Guest Session", now it's under guest session and the display is normal. 3. switch back to your user account, then switch to guest session again. 4. The screen remains blank. I can't reproduce this issue under Precise. Vivid snapshot 22-Mar-2015 can also reproduce this issue. Package: lightdm 1.10.3-0ubuntu2 unity-settings-daemon-14.04.0+14.10.20140606~0ubuntu1 --- ApportVersion: 2.14.1-0ubuntu3.2 Architecture: amd64 CurrentDesktop: Unity DistributionChannelDescriptor: # This is a distribution channel descriptor # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor canonical-oem-stella-datong140729-kylin-trusty-amd64-20150205-2 DistroRelease: Ubuntu 14.04 InstallationDate: Installed on 2015-02-09 (1 days ago) InstallationMedia: Ubuntu 14.04 "Trusty" - Build amd64 LIVE Binary 20150205-07:08 LightdmConfig: [SeatDefaults] autologin-guest=false autologin-user-timeout=0 autologin-session=lightdm-autologin Package: lightdm 1.10.3-0ubuntu2 PackageArchitecture: amd64 ProcVersionSignature: Ubuntu 3.13.0-38.65stella1-generic 3.13.11.8 Tags: trusty third-party-packages Uname: Linux 3.13.0-38-generic x86_64 UpgradeStatus: No upgrade log present (probably fresh install) UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo _MarkForUpload: True To manage notifications about this bug go to: https://bugs.launchpad.net/oem-priority/+bug/1405349/+subscriptions -- Mailing list: https://launchpad.net/~dx-packages Post to : dx-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~dx-packages More help : https://help.launchpad.net/ListHelp
