Windows on external monitor are inaccessible after disconnecting HDMI

[Wes]

To reproduce:

 - Connected to 1920x1080 monitor HDMI (LG 22MC57HQ-P)
 - Dragged windows to monitor display
 - Disconnected HDMI from laptop after resuming locked session

Program windows that were on the monitor display stayed off the screen after disconnecting, could not be dragged back even with GNOME dock -> "All Windows" displaying the preview. Had to reconnect to the monitor and manually drag them back to the laptop screen.

It sounds like others have had this issue:
https://askubuntu.com/questions/665709/how-to-recover-offscreen-window-after-disconnecting-second-monitor
https://superuser.com/questions/382165/connect-and-disconnect-a-monitor-in-ubuntu/1215472#1215472

Expected: disconnect from external monitor detected by windowing system, always moves all windows to the active screen automatically.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: compiz 1:0.9.13.1+18.04.20180302-0ubuntu1
ProcVersionSignature: User Name 4.15.0-39.42-generic 4.15.18
Uname: Linux 4.15.0-39-generic x86_64
.tmp.unity_support_test.0:

ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
BootLog: /dev/sda4: clean, 227526/1343488 files, 1348444/5371136 blocks
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CompositorRunning: None
CurrentDesktop: ubuntu:GNOME
Date: Wed Nov 28 16:10:35 2018
DistUpgraded: 2018-09-28 16:11:26,468 DEBUG icon theme changed, re-reading
DistroCodename: bionic
DistroVariant: ubuntu
DkmsStatus:
 virtualbox, 5.2.10, 4.15.0-38-generic, x86_64: installed
 virtualbox, 5.2.10, 4.15.0-39-generic, x86_64: installed
EcryptfsInUse: Yes
GraphicsCard:
 Intel Corporation Haswell-ULT Integrated Graphics Controller [8086:0a16] (rev 0b) (prog-if 00 [VGA controller])
   Subsystem: Dell Haswell-ULT Integrated Graphics Controller [1028:061f]
GsettingsChanges:
 b'org.compiz.core' b'hsize' b'2'
 b'org.compiz.core' b'active-plugins' b"['core', 'composite', 'opengl', 'wall', 'grid', 'vpswitch', 'place', 'session', 'move', 'imgpng', 'snap', 'compiztoolbox', 'regex', 'mousepoll', 'resize', 'animation', 'workarounds', 'expo', 'fade', 'scale', 'ezoom', 'unityshell', 'switcher']"
 b'org.compiz.core' b'vsize' b'2'
InstallationDate: Installed on 2018-09-12 (77 days ago)
InstallationMedia: Ubuntu 16.04.5 LTS "Xenial Xerus" - Release amd64 (20180731)
MachineType: Dell Inc. Latitude 3340
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-39-generic root=UUID=1c6a1916-ac97-4bdf-8f15-14d986e621a2 ro
SourcePackage: compiz
UpgradeStatus: Upgraded to bionic on 2018-09-28 (61 days ago)
dmi.bios.date: 07/09/2018
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A17
dmi.board.vendor: Dell Inc.
dmi.chassis.type: 9
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA17:bd07/09/2018:svnDellInc.:pnLatitude3340:pvr00:rvnDellInc.:rn:rvr:cvnDellInc.:ct9:cvr:
dmi.product.name: Latitude 3340
dmi.product.version: 00
dmi.sys.vendor: Dell Inc.
version.compiz: compiz 1:0.9.13.1+18.04.20180302-0ubuntu1
version.libdrm2: libdrm2 2.4.91-2
version.libgl1-mesa-dri: libgl1-mesa-dri 18.0.5-0ubuntu0~18.04.1
version.libgl1-mesa-glx: libgl1-mesa-glx 18.0.5-0ubuntu0~18.04.1
version.xserver-xorg-core: xserver-xorg-core 2:1.19.6-1ubuntu4.2
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.10.5-1ubuntu1
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:18.0.1-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20171229-1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.15-2

[Wes (wesinator)]

I've also noticed that after disconnecting, if I drag windows to the right, they go off the laptop screen and become inaccessible from the screen.

So it seems it isn't properly detecting that the HDMI is disconnected (from laptop)

This is an extremely annoying and astonishing usability issue.

Expected: on disconnect, windowing reset to normal primary mode, moving all windows back to the active screen

Workaround: opening Displays Settings forces reset