Bug#1121628: systemd: v259~rc1 causes GNOME Xwayland integration to regress, affecting themes and scaling

[Simon McVittie]

On Sat, 29 Nov 2025 at 19:34:27 -0500, Jeremy Bícha wrote:
>I can reproduce this issue in Debian Testing, but I can't reproduce
>this issue after downgrading systemd back to 258.1-2.
>
>See also the upstream GNOME issue:
>https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8866

This also affects the mechanism that communicates themes via Xsettings. 
An easy way to see it is to run a GTK 2 application such as gmpc, in a 
GNOME Wayland desktop session, with the gnome-themes-extra package installed.

Bad result: GTK 2's ugly internal default theme, "Raleigh"
(Windows-95-inspired, with thick pseudo-3D bevels)

Good result: GNOME's Adwaita theme, looking superficially similar to GTK 
3 or 4

Workaround: systemctl start --user gnome-session-x11-services-ready.target

Unlike the failure mode involving scaling, reproducing the problem this 
way this does not require a highDPI display.

GNOME upstream says:

>The services behind that target should only be started when Xwayland is
>running. Indeed, starting the services unconditionally would result in
>Xwayland being launched as well, even if no actual X11 client is running.
>
>That is why it isn't started automatically by systemd.
>
>If an X11 client is started, mutter launches Xwayland and emits a
>signal. In response to that signal, gnome-shell asks systemd to start
>the unit.
>
>So that must be where the bug is after all, either mutter doesn't emit
>the signal, or gnome-shell fails to start the systemd target.

Given Jeremy's observation that this worked with 258.1-2 but doesn't work 
with 259~rc1, presumably there must have been a behaviour change in 
systemd that changed how this happens. This could either be a systemd 
bug, or a gnome-shell/mutter bug that happens to have been harmless 
until now.

     smcv