Since KDE/plasma updates a few weeks back, KDE seems to no longer correctly detect when my external monitor (which I set to primary, when it's connected) is disconnected. Even rebooting does not correct the problem.
The only workaround is to delete the contents of /.local/share/kscreen/ and restart the desktop.
This was working fine in earlier versions of the plasma 5 desktop and kde 5 frameworks with the same hardware.
ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: kscreen 4:5.2.1-0ubuntu1
ProcVersionSignature: Ubuntu 3.18.0-13.14-generic 3.18.5
Uname: Linux 3.18.0-13-generic x86_64
ApportVersion: 2.16.1-0ubuntu2
Architecture: amd64
CurrentDesktop: KDE
Date: Thu Feb 26 07:54:12 2015
SourcePackage: kscreen
UpgradeStatus: No upgrade log present (probably fresh install)
It looks like this has improved somewhat: It's no longer necessary to delete the contents of .local/
The problem where the system does not appropriately act upon connection/
Scenario 1:
1. Connected to external (primary) 2560x1080 monitor, put laptop to sleep.
2. Disconnect monitor.
3. Wake laptop.
4. kscreenlocker_greet is unresponsive, and trying to draw a 2560x1080 unlock screen on the laptop's 1920x1080 screen.
5. The only way to get past this is to go to a terminal and kill the locker process (which will respawn with correct screen info). The correct screen setup will be used after you unlock the respawned locker.
Scenario 2:
1. With laptop not connected to monitor, sleep system.
2. Connect to external 2560x1080 monitor.
3. Wake laptop.
4. After unlocking screen, desktop resolution is still 1920x1080, filling only the left 3/4ths of the screen.