KDE Plasma: Flatpak applications ignore appearance settings with xdg-desktop-portal 1.17.2-1ubuntu1

The issue comes from the new xdg-desktop-portal, see https://github.com/flatpak/xdg-desktop-portal/pull/1082 for details

The new configuration system gives more flexibility to the desktop environment to select the portal variants that should be used but it might require tweaks from those desktop environments

The GNOME configuration which fixes the issue was added in
https://salsa.debian.org/gnome-team/gnome-session/-/commit/b201c9c4
which is including in the gnome-session version uploaded to Ubuntu yesterday

This bug was fixed in the package xdg-desktop-portal-kde - 5.27.7-1ubuntu2

---------------
xdg-desktop-portal-kde (5.27.7-1ubuntu2) mantic; urgency=medium

  * Add upstream_install-portals.conf-for-plasma.patch to adapt to
    changes in xdg-desktop-portal 1.17.x (LP: #2034763)

 -- Rik Mills <email address hidden> Sat, 09 Sep 2023 19:39:21 +0100

That didn't solve the issue for me, but downgrading xdg-desktop-portal package still does.

I haven't tested Wayland/X11 behaviour difference earlier, but what I've noticed now is that it only affects the Wayland apps. For example, Firefox under XWayland (without overrides) looks normal, but after I add the following override

flatpak override --user --env="MOZ_ENABLE_WAYLAND=1" org.mozilla.firefox

the issue manifests itself.

Alexey, are you able to open a new bug for that issue?

You can use

ubuntu-bug xdg-desktop-portal-kde