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

[Anjune M]

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, without having to kill and re-run plasmashell

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.

[Anjune M (anj-tuesday)]

Bug no longer seems to affect me as of (roughly) April 6.