gnome-shell/Xorg high CPU and log flooded with: Window manager warning: Overwriting existing binding of keysym [when using fcitx]

[compul]

I installed Ubuntu and everything worked fine.
Suddenly, when changing my input language from English to German via alt+space, gnome-shell froze. After a reboot I can login, but am greeted by a cursor on a black background, with gnome-shell and Xorg eating one of my CPU cores.
i3wm starts just fine.

*edit*

After doing the following, gnome-shell will start again:

gsettings list-schemas | xargs -n 1 gsettings reset-recursively
systemctl restart gdm

When switching to the German keyboard layout again, it will crash. journalctl is filled with warnings of the following type:

gnome-shell: Window manager warning: Overwriting existing binding of keysym XX with keysym XX (keycode Y)

*edit*
I noticed that the issue disappears when exiting Fcitx. So maybe it's an issue with Fcitx instead of gnome-shell.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: gnome-shell 3.38.2-1ubuntu1~20.10.1
ProcVersionSignature: Ubuntu 5.8.0-45.51-generic 5.8.18
Uname: Linux 5.8.0-45-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu50.5
Architecture: amd64
CasperMD5CheckResult: skip
Date: Tue Mar 23 14:54:28 2021
DisplayManager: gdm3
InstallationDate: Installed on 2021-03-22 (1 days ago)
InstallationMedia: Ubuntu 20.10 "Groovy Gorilla" - Release amd64 (20201022)
RelatedPackageVersions: mutter-common 3.38.2-1ubuntu1~20.10.1
SourcePackage: gnome-shell
UpgradeStatus: No upgrade log present (probably fresh install)

[Daniel van Vugt (vanvugt)]

The log probably says it all:

[ 359.469600] Rupert gnome-shell[2673]: Window manager warning: Overwriting existing binding of keysym 40 with keysym 71 (keycode 18).
[ 359.484328] Rupert gnome-shell[2673]: Window manager warning: Overwriting existing binding of keysym 40 with keysym 71 (keycode 18).
[ 359.500056] Rupert gnome-shell[2673]: Window manager warning: Overwriting existing binding of keysym 40 with keysym 71 (keycode 18).
[ 359.515346] Rupert gnome-shell[2673]: Window manager warning: Overwriting existing binding of keysym 40 with keysym 71 (keycode 18).
[ 359.530289] Rupert gnome-shell[2673]: Window manager warning: Overwriting existing binding of keysym 40 with keysym 71 (keycode 18).
[ 359.546892] Rupert gnome-shell[2673]: Window manager warning: Overwriting existing binding of keysym 40 with keysym 71 (keycode 18).
...

[Daniel van Vugt (vanvugt)]

See also bug 1857392.

[Daniel van Vugt (vanvugt)]

Upstream bug https://gitlab.gnome.org/GNOME/mutter/-/issues/1409 sounds similar to this.

[compul (compul)]

I uninstalled fcitx and started using ibus instead. The Window manager warning still appears, but only once. No freeze happens, and I can feel no lag either. So fcitx must be causing some sort of loop.