GNOME Magnifier Flickering when using none-full screen modes

[Lee Clark]

When using the GNOME magnifier (Zoom) feature the magnified display flickers when used in any of the following none full screen modes: Left, Right, Top or Bottom screen part. This occurs when the mouse moves towards the edge directly opposite to the magnified view. This also occurs if 'Magnifier extends outside of screen' is enabled.

To reproduce:
* Open GNOME Control Centre.
* Go to Universal Access.
* Click on Zoom.
* Enable Zoom
* Selct either Top, Bottom, Left or Right from the screen part drop down list.
* Move the mouse cursor to the opossite edge of the screen to that where the magnified view is located.

I have tested on multiple systems running Ubuntu 18.04, 18.10, 19.04 and Fedora 30.

Tested with all extension uninstalled.

Happy to provide additional details.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gnome-shell 3.28.3+git20190124-0ubuntu18.04.2
ProcVersionSignature: Ubuntu 4.18.0-21.22~18.04.1-generic 4.18.20
Uname: Linux 4.18.0-21-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.9-0ubuntu7.6
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu May 30 19:14:43 2019
DisplayManager: gdm3
ExecutablePath: /usr/bin/gnome-shell
InstallationDate: Installed on 2019-03-12 (78 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
ProcEnviron:
 LANGUAGE=en_GB:en
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-shell
UpgradeStatus: No upgrade log present (probably fresh install)

[Lee Clark (lclark125)]

Tested in an 19.04 VM and the issue deos not appear to be present. I will test on real hardware.

[Lee Clark (lclark125)]

Tested on physical hardware with 19.04. The issue is still present but the viaul manifestaton is slightly different. Rather than black flickers it's more of a shading effect that flickers on and off with mouse movement. It can also be caused by enabling the option to extent the magnified contents beyond the bounds of the desktop.

[Daniel van Vugt (vanvugt)]

I can't yet reproduce this bug myself...

Please:

1. Run:

 lspci -k > lspcik.txt

and send us the file 'lspcik.txt'.

2. Uninstall your extensions:

 '<email address hidden>', '<email address hidden>'

then reboot and tell us if the problem still occurs. We find a large number of bugs are caused by extensions so need to know if the bug still happens without them.

[Lee Clark (lclark125)]

I've attached the requested output.

I have done additional testing today on my Dell XPS laptop running Ubuntu 19.04, my personal desktop running 18.04 and my work PC running 18.04.

* I removed all extensions, uninstalled packages for those that had them or deleted via the gnome extension website.
* Rebooted
* Re-enabled magnifier and switched between 'Full Screen' and 'Left' Screen parts.

I see the issue on all three machines though on 19.04 the effect is slightly different.

Opening the activities overlay causes the issue to fanish until it is closed again.

[Lee Clark (lclark125)]

Here is the lspci output from my laptop.

[Lee Clark (lclark125)]

OK, turns out my laptop is on 18.10 not 19.04. I assumed the upgrade would go all the way but I was wrong. :(

I'm upgrading now and will test again.

[Lee Clark (lclark125)]

Both of my machines are now on 19.04, I'm still seeing the issue. I've got all extensions turned off.

[Lee Clark (lclark125)]

I couldn't figure out why I didn't see the issue in my VM. But additional testing shows the issue in the VM. It depends on which windows are visible. If you close all windows and return to the desktop it always occurs.

Also enabling the 'Magnifier extends outside screen' checkbox also causes the issues.

[Daniel van Vugt (vanvugt)]

Please ensure all extensions are uninstalled and not just turned off. Extensions still interfere even when they are turned off.

[Lee Clark (lclark125)]

Completely removedd all extensions include those shipped with Ubuntu 19.04. Issue still occurs. Also tested with a clean install in VM.

[Lee Clark (lclark125)]

Also tested on Fedora 30, the issue exists but manifests slightly differently.

Should I raise this upstream? If so which GNOME project should it be raised against?

[Daniel van Vugt (vanvugt)]

Oh yes. I can reproduce the bug now. I didn't know partial screen magnification was even an option.

Please report the bug here:

https://gitlab.gnome.org/GNOME/mutter/issues

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Lee Clark (lclark125)]

Upstream bug is here: https://gitlab.gnome.org/GNOME/mutter/issues/625

[Lee Clark (lclark125)]

This bug appears to have been fixed at some point in the development of gnome 3.6, still present in 3.4 on Ubutnu 19.10 and 18.04.