[Bug 1760319] [NEW] Widgets and desktop/wallpaper gone after waking from screen timeout if PLASMA_USE_QT_SCALING=1

[anj]

Public bug reported:

Ubuntu release:

Ubuntu Bionic Beaver (development branch); the problem also occurs in
Kubuntu 17.10, if upgraded to Plasma 5.12.x

Package version (plasma-workspace):

  Installed: 4:5.12.4-0ubuntu2
  Candidate: 4:5.12.4-0ubuntu2
  Version table:
 *** 4:5.12.4-0ubuntu2 500
        500 http://de.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status

What I expect to happen:

To continue where I left off after waking up my screen

What happens instead:

When the screen (external monitor, not a laptop) turns off due to a
timeout, whether this was set in Plasma's power management settings or
using xset dpms <whatever>, and I wake it back up again to continue
working/faffing around online, any widgets have vanished and the desktop
is a solid, unresponsive black.

The rightmost edge of my panel is still visible in the top left corner
of the screen.

"Hot corners" and compositing still work; windows are still decorated.

Apport is silent.

This does not happen if I wake the screen back up within the first
couple seconds, but after ten seconds or so it happens reliably every
time.

It also does not happen when I don't set PLASMA_USE_QT_SCALING=1 (but I
use a Hi-DPI screen and would rather have Plasma scaled correctly).

Using Xorg. Proprietary Nvidia driver, tested with 384 and all 390 point
releases.

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

** Attachment added: "Screenshot of what it looks like"
   https://bugs.launchpad.net/bugs/1760319/+attachment/5097047/+files/Screenshot_20180331_174125.png

** Summary changed:

- Widgets and desktop/wallpaper gone after screen timeout if PLASMA_USE_QT_SCALING=1
+ Widgets and desktop/wallpaper gone after waking from screen timeout if PLASMA_USE_QT_SCALING=1

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https://bugs.launchpad.net/bugs/1760319

Title:
  Widgets and desktop/wallpaper gone after waking from screen timeout if
  PLASMA_USE_QT_SCALING=1

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