Firefox UI crashes after mpv bypasses kwin(?) compositor

[John Uren]

Firefox's UI crashes when I play a fullscreen video on mpv. I've tracked it down so far to the kubuntu compositor option (kwin?).

I would expect to be able to use Firefox after playing a fullscreen video with mpv. What happens is that after playing the video and exiting mpv, firefox's UI freezes. The most obvious effect is that I can't open new tabs in firefox.

Steps to recreate bug in Kubuntu 21.04:

1) Enable the "Allow applications to block compositing". I'm not sure if this is in by default.
2) Open firefox
3) Start a video in mpv.
4) Enable fullscreen in media player.
5) Stop the video
6) Leave fullscreen playback
7) (Try to) Open a new tab in firefox. The GUI doesn't show the new tab and looks like it crashes.

If you close Firefox at this point, you still get the pop-up about closing multiple tabs.

I've been able to work out that disabling the "Allow applications to block compositing" option "fixes" this at the cost of choppier video playback with 50fps videos.

VLC and Dragon Player doesn't trigger the crash.

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: firefox 88.0.1+build1-0ubuntu0.21.04.2
ProcVersionSignature: Ubuntu 5.11.0-16.17-generic 5.11.12
Uname: Linux 5.11.0-16-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.11-0ubuntu65
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: john 1596 F.... pulseaudio
 /dev/snd/controlC0: john 1596 F.... pulseaudio
BuildID: 20210504152106
CasperMD5CheckResult: unknown
Channel: Unavailable
CurrentDesktop: KDE
Date: Wed May 12 11:35:22 2021
DefaultProfileExtensions: extensions.sqlite corrupt or missing
DefaultProfileIncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
DefaultProfileLocales: extensions.sqlite corrupt or missing
DefaultProfilePrefErrors: Unexpected character ',' before close parenthesis @ /usr/lib/firefox/omni.ja:greprefs.js:352
DefaultProfileThemes: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
Profile0Extensions: extensions.sqlite corrupt or missing
Profile0IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
Profile0Locales: extensions.sqlite corrupt or missing
Profile0PrefErrors: Unexpected character ',' before close parenthesis @ /usr/lib/firefox/omni.ja:greprefs.js:352
Profile0PrefSources: prefs.js
Profile0Themes: extensions.sqlite corrupt or missing
Profile2Extensions: extensions.sqlite corrupt or missing
Profile2IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
Profile2Locales: extensions.sqlite corrupt or missing
Profile2PrefErrors: Unexpected character ',' before close parenthesis @ /usr/lib/firefox/omni.ja:greprefs.js:352
Profile2PrefSources: prefs.js
Profile2Themes: extensions.sqlite corrupt or missing
Profiles:
 Profile1 (Default) - LastVersion=None/None (Out of date)
 Profile0 - LastVersion=83.0/20201112153044 (Out of date)
 Profile2 - LastVersion=88.0.1/20210504152106 (In use)
RebootRequiredPkgs: linux-image-5.11.0-17-generic
RunningIncompatibleAddons: False
SourcePackage: firefox
UpgradeStatus: Upgraded to hirsute on 2021-05-07 (4 days ago)
dmi.bios.date: 03/17/2021
dmi.bios.release: 1.14
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.14.3
dmi.board.name: 0F6P3V
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 10
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr1.14.3:bd03/17/2021:br1.14:svnDellInc.:pnXPS139370:pvr:rvnDellInc.:rn0F6P3V:rvrA00:cvnDellInc.:ct10:cvr:
dmi.product.family: XPS
dmi.product.name: XPS 13 9370
dmi.product.sku: 07E6
dmi.sys.vendor: Dell Inc.