stretched workspace preview after unpluging secondary display

When you plug a secondary display as extended desktop, you will see both of them in viewport switch preview and it's OK. But when you unplug the secondary display, the size of previews size doesn't change and the viewport will be stretched in order to fit in the wide sized preview.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: unity 7.3.2+15.10.20151002.2-0ubuntu1
ProcVersionSignature: Ubuntu 4.2.0-15.18-generic 4.2.3
Uname: Linux 4.2.0-15-generic x86_64
.tmp.unity.support.test.0:

ApportVersion: 2.19.1-0ubuntu2
Architecture: amd64
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CompositorRunning: compiz
CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0'
CompositorUnredirectFSW: true
CurrentDesktop: Unity
Date: Mon Oct 12 23:39:38 2015
DistUpgraded: 2015-09-05 22:58:30,894 DEBUG enabling apt cron job
DistroCodename: wily
DistroVariant: ubuntu
GraphicsCard:
 Intel Corporation 3rd Gen Core processor Graphics Controller [8086:0166] (rev 09) (prog-if 00 [VGA controller])
   Subsystem: Lenovo Device [17aa:21f3]
InstallationDate: Installed on 2015-04-24 (171 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
MachineType: LENOVO 2349KEG
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.2.0-15-generic root=UUID=a1abeb16-2852-43c8-8d5b-330faa072bf8 ro ipv6.disable=1 quiet splash vt.handoff=7
SourcePackage: unity
UdevLog: Error: [Errno 2] No such file or directory: '/var/log/udev'
UpgradeStatus: Upgraded to wily on 2015-09-05 (37 days ago)
dmi.bios.date: 09/04/2013
dmi.bios.vendor: LENOVO
dmi.bios.version: G1ET97WW (2.57 )
dmi.board.asset.tag: Not Available
dmi.board.name: 2349KEG
dmi.board.vendor: LENOVO
dmi.board.version: Win8 Pro DPK TPG
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Not Available
dmi.modalias: dmi:bvnLENOVO:bvrG1ET97WW(2.57):bd09/04/2013:svnLENOVO:pn2349KEG:pvrThinkPadT430:rvnLENOVO:rn2349KEG:rvrWin8ProDPKTPG:cvnLENOVO:ct10:cvrNotAvailable:
dmi.product.name: 2349KEG
dmi.product.version: ThinkPad T430
dmi.sys.vendor: LENOVO
version.compiz: compiz 1:0.9.12.2+15.10.20151002-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.64-1
version.libgl1-mesa-dri: libgl1-mesa-dri 11.0.2-1ubuntu3
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 11.0.2-1ubuntu3
version.xserver-xorg-core: xserver-xorg-core 2:1.17.2-1ubuntu9
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.9.2-1ubuntu1
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.5.0+git20150819-0ubuntu1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20150808-0ubuntu3
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.11-1ubuntu3
xserver.bootTime: Mon Oct 12 17:19:14 2015
xserver.configfile: default
xserver.errors:

xserver.logfile: /var/log/Xorg.0.log
xserver.outputs:
 product id 12604
 vendor AUO
xserver.version: 2:1.17.2-1ubuntu9