Same-application window switcher resizes the application windows

[Giuseppe D'Angelo]

The scenario is this:

* Multiple windows of the same application are opened
* Some of them are fullscreen
* The user clicks the application icon in the side tray, starting the window switcher for it (so that the user can select a certain window)

This last step actually triggers a *resize* for the fullscreened windows (for no reason at all): the windows get shrunk vertically a little bit, and regain their fullscreen size when the user makes a decision.

This might look like a side issue, but it's not: a fullscreen Firefox (or any other browser) will relayout the current webpage because of that size change; complex GUI applications like Blender will start their complicated heuristics to optimize their UI controls; and so on.

For these applications, there's a huge spike in CPU when triggering the window switcher (CPU used by the applications! Not by Unity or Compiz), while such switching should be totally transparent to the applications.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: unity 7.4.0+16.04.20160906-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-75.96-generic 4.4.59
Uname: Linux 4.4.0-75-generic x86_64
.tmp.unity_support_test.0:

ApportVersion: 2.20.1-0ubuntu2.5
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: Wed May 3 00:53:41 2017
DistUpgraded: 2016-09-29 11:49:27,678 WARNING no activity on terminal for 300 seconds (Applicazione delle modifiche)
DistroCodename: xenial
DistroVariant: ubuntu
GraphicsCard:
 Intel Corporation Broadwell-U Integrated Graphics [8086:1616] (rev 09) (prog-if 00 [VGA controller])
   Subsystem: Lenovo Broadwell-U Integrated Graphics [17aa:2225]
InstallationDate: Installed on 2015-10-15 (565 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
MachineType: LENOVO 20E2000PUK
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.4.0-75-generic.efi.signed root=UUID=daffac49-1ba4-415c-9de1-a1d41e8e4c0f ro quiet splash vt.handoff=7
SourcePackage: unity
UpgradeStatus: Upgraded to xenial on 2016-09-29 (215 days ago)
dmi.bios.date: 08/20/2015
dmi.bios.vendor: LENOVO
dmi.bios.version: N11ET34W (1.10 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20E2000PUK
dmi.board.vendor: LENOVO
dmi.board.version: SDK0E50510 WIN
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.modalias: dmi:bvnLENOVO:bvrN11ET34W(1.10):bd08/20/2015:svnLENOVO:pn20E2000PUK:pvrThinkPadW550s:rvnLENOVO:rn20E2000PUK:rvrSDK0E50510WIN:cvnLENOVO:ct10:cvrNone:
dmi.product.name: 20E2000PUK
dmi.product.version: ThinkPad W550s
dmi.sys.vendor: LENOVO
version.compiz: compiz 1:0.9.12.2+16.04.20160823-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.70-1~ubuntu16.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 12.0.6-0ubuntu0.16.04.1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 12.0.6-0ubuntu0.16.04.1
version.xserver-xorg-core: xserver-xorg-core 2:1.18.4-0ubuntu0.2
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.10.1-1ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.7.0-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20160325-1ubuntu1.2
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.12-1build2
xserver.bootTime: Tue May 2 01:18:35 2017
xserver.configfile: default
xserver.errors: open /dev/dri/card0: No such file or directory
xserver.logfile: /var/log/Xorg.0.log
xserver.outputs:
 product id 38562
 vendor MEI
xserver.version: 2:1.18.4-0ubuntu0.2

[Giuseppe D'Angelo (dangelo)]

Qt-based testcase is attached.

Instructions:
* Unpack, qmake, make
* Run the application at least twice, making at least one fullscreened
* Trigger the window switcher by clicking on the Unity's sidebar

The fullscreened one will print a wrong resize event. E.g.:

Resized to QResizeEvent(2880, 1572) <- fullscreened
Changed to QEvent(ActivationChange, 0x7ffede2bfa30)
Changed to QEvent(ActivationChange, 0x7ffede2bfa30)
Resized to QResizeEvent(2880, 1516) <- task switcher activated
Resized to QResizeEvent(2880, 1516) <- no idea
Changed to QEvent(ActivationChange, 0x7ffede2bfa30)
Changed to QEvent(ActivationChange, 0x7ffede2bfa30)
Resized to QResizeEvent(2880, 1572) <- exit from task switcher
Resized to QResizeEvent(2880, 1572)

(comments mine)