Alt+Tab produces visual artefacts when a modal dialog is open [with mismatched mutter packages installed]

Thanks for the bug report. I am also using

  gnome-shell 3.28.3+git20190124-0ubuntu18.04.1
  mutter 3.28.3+git20190124-0ubuntu18.04.1

but cannot reproduce this bug. In particular I can't see any option to "0. Change the default Alt+Tab to switch windows".

Please:

 1. Detail how to "0. Change the default Alt+Tab to switch windows".

 2. Try uninstalling all your extensions and tell us if the bug still happens:

 "b'org.gnome.shell' b'enabled-extensions' b"['<email address hidden>', '<email address hidden>', '<email address hidden>', '<email address hidden>', '<email address hidden>', '<email address hidden>', '<email address hidden>', '<email address hidden>']"

To change Alt+Tab behaviour:

1. Open Settings → Devices → Keyboard → Keyboard Shortcuts
2. Scroll down to Navigation, you will find the default for "Switch applications" is Alt+Tab
3. Change "Switch applications" to Super+Tab
4. Change "Switching windows" to Alt+Tab

I can still reproduce the bug after disabling all extensions.

Thanks. I still can't reproduce the problem using that alternate switcher.

Can you please try uninstalling all extensions and then rebooting? Just disabling them doesn't completely remove them from gnome-shell's code paths unfortunately.

Please also run:

  dpkg -l > allpackages.txt

and send us the file 'allpackages.txt'.

Oh, are you using multiple monitors at all? If so then it would be the same problem as was fixed in https://gitlab.gnome.org/GNOME/mutter/merge_requests/511

I purged all extensions, restarted and I can still reproduce the bug.

No, I don't use multi-monitors.

Remember that you have to start Alt+Tabbing from the modal dialogue, if you start switching from a normal window, you will not reproduce the bug.

The file is attached.

Thanks.

Please try these separately (and reboot after each one) in /etc/environment:

  CLUTTER_PAINT=disable-culling

or

  CLUTTER_PAINT=disable-offscreen-redirect

CLUTTER_PAINT=disable-culling: no change, bug is reproducible

CLUTTER_PAINT=disable-offscreen-redirect: no change, bug is reproducible

Could you try if that's an issue in disco for you? what videocard/driver do you use?

I'm using Intel UHD Graphics 620 (Kabylake GT2).

Here is another severe screencast on Bionic, other windows appear momentarily behind the modal dialogue while switching.
Note that you have to start switching from the modal dialogue to trigger the bug, and Alt+Tab has to be configured to switch windows not applications. I believe the default beyond Bionic is for Alt+Tab to switch windows.

Hum, is that issue specific to the updates in proposed? If so it's likely a duplicate of the issue discussed on the SRU bug #1811900

I cannot reproduce this bug in an up-to-date Disco.

OK, I think I discovered something, when I updated mutter in bionic-proposed, it didn't pull the updated version of libmutter-2-0 and gir1.2-mutter-2. So I had a mix of:
mutter 3.28.3+git20190124-0ubuntu18.04.1
mutter-common 3.28.3+git20190124-0ubuntu18.04.1
gir1.2-mutter-2 3.28.3-2~ubuntu18.04.2
libmutter-2-0 3.28.3-2~ubuntu18.04.2

Now I cannot reproduce this bug after I manually installed libmutter-2-0 and gir1.2-mutter-2 from bionic-proposed.

So I think the problem is mixing old and new versions of mutter packages. This bug could be closed unless there is a need not to.

Thanks everyone.

That's still a bug because it shouldn't be possible to successfully install different packages which cause bugs like this. But it's also a bug that we are unlikely to ever encounter again or fix.

I found the upstream bug and attached it. The bug symptoms are the same.

Thanks. I'm glad to have fixed it already then :)

https://gitlab.gnome.org/GNOME/mutter/merge_requests/117