multi display mirror mode is ignored and enabled wrongly when external monitor is added or wakes

[spike speigel]

When an external monitor is plugged in or simply wakes from sleep the following happens erroneously:

The "Mirror displays" setting is ignored and enabled even though it was previously disabled.

The "Mirror displays" setting checkbox is unchecked even though the system wrongly enabled the setting upon the monitor waking.

I am forced to toggle the "Mirror displays" setting so that it is disabled once again so that I can reconfigure my monitors after they wake.

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: xfce4-settings 4.12.1-1ubuntu1
ProcVersionSignature: Ubuntu 4.13.0-10.11-generic 4.13.1
Uname: Linux 4.13.0-10-generic x86_64
ApportVersion: 2.20.7-0ubuntu1
Architecture: amd64
CurrentDesktop: XFCE
Date: Tue Sep 12 19:30:15 2017
ExecutablePath: /usr/bin/xfce4-settings-manager
InstallationDate: Installed on 2017-04-29 (136 days ago)
InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Beta amd64 (20170408)
ProcEnviron:
 LANGUAGE=en_US
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: xfce4-settings
UpgradeStatus: No upgrade log present (probably fresh install)