[Bug 1575653] [NEW] Plasma 5.5.5 breaks on multi-monitor when the displays go into standby

[Piotr Michniewski]

Public bug reported:

$ lsb_release -rd
Description:    Ubuntu 16.04 LTS
Release:        16.04


$ apt-cache policy plasma-workspace
plasma-workspace:
  Installed: 4:5.5.5.2-0ubuntu1
  Candidate: 4:5.5.5.2-0ubuntu1
  Version table:
 *** 4:5.5.5.2-0ubuntu1 500
        500 http://pl.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        100 /var/lib/dpkg/status

Steps to reproduce:
1. Connect two monitors to PC
2. Configure them in KScreen to be side-by-side with second one set to primary
3. Configure Plasma desktops on the screens, add a panel to both of them
4. Lock screen
5. Wait for the screens to into standby
6. Unlock screen

Expected results:
Properly running desktop with everything just like it was before locking the screen.

Actual results:
Plasma randomly rearranged desktops (folder view that was on second screen, now is on first) and widgets (one of the panels went missing).

I can see the following logs in .xsession-errors:

kscreen: Primary output changed from KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" ) to KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" )
kscreen: Primary output changed from KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" ) to KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" )
kscreen: Failed to retrieve current config:  "Backend invalidated"
kscreen: Primary output changed from KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" ) to KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" )
kscreen: Primary output changed from KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" ) to KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" )
kscreen: Primary output changed from KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" ) to KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" )
kscreen: Primary output changed from KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" ) to KScreen::Output(Id: 68 , Name: "HDMI2" ) ( "HDMI2" )
kscreen.kded: Change detected
kscreen.kded: Saving current config to file
kscreen.kded: Calculating config ID for KScreen::Config(0x2071280)
kscreen.kded:   Part of the Id:  "299872f79619431d6101339eae453e66"
kscreen.kded:   Part of the Id:  "9b5d9fa663a36c77d7b7210c2c7200b8"
kscreen.kded:   Config ID: "896ed53d3eb2957d9577f10fecf9bba3"
kscreen.kded: Config saved on:  "/home/pmichniewski/.local/share/kscreen/896ed53d3eb2957d9577f10fecf9bba3"

The issue can be fixed by restarting plasmashell but I have to do this
every time the screen is unlocked after going into standby.

** Affects: plasma-workspace (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1575653

Title:
  Plasma 5.5.5 breaks on multi-monitor when the displays go into standby

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