Bug#868471: kwin: "Shade" action glitches "Plastik" decorations

[mdmilosz <mdmilosz@outlook.com>]

Source: kwin
Severity: normal

Dear Maintainer,

I noticed that using the "Shade" action with the "Plastik" decorations
leads to a graphical glitch:

http://i.imgur.com/fQINOFJ.png

The action is reversible. The windows cannot be "shaded" without causing
the glitch.

I've asked a fellow Debian user and they confirmed the bug happens for them
as well.

The glitch doesn't happen with the "Breeze" decorations:

http://i.imgur.com/BytOM2B.png


-- System Information:
Debian Release: buster/sid
  APT prefers unstable
  APT policy: (900, 'unstable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.11.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=pl_PL.UTF-8, LC_CTYPE=pl_PL.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB:en_US (charmap=UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)