tl;dr: If you want to be sure your application will be displayed with
the correct icon under different Wayland compositors make sure that
your GApplication (or GtkApplication) uses
g_set_prgname(your_g_application_id);
on GTK+3. On GTK+4 this is handled for you.
Details: While working on touch based window switching for the
Librem5 I noticed that lots of the GNOME application did not end up
with a proper icon when using g_desktop_app_info_new (desktop_id).
The desktop_id is determined from the Wayland xdg surface's app_id
as specified by in Wayland's xdg-shell protocol.
The protocol says:
The compositor shell will try to group application surfaces together
by their app ID. As a best practice, it is suggested to select app
ID's that match the basename of the application's .desktop file.
For example, "org.freedesktop.FooViewer" where the .desktop file is
"org.freedesktop.FooViewer.desktop".
It's even more explicit about the relation of the app_id to the D-Bus service name:
For D-Bus activatable applications, the app ID is used as the D-Bus
service name.
So why does this currently fail? It's because GTK+3 historically uses
g_get_prgname() to set the app_id and this defaults to
application's basename. But what we rather want is
g_application_id == D-Bus service name == $(basename desktop_file_path .desktop) == xdg app_id
There were patches by Jonas Ådahl to fix this but those were partially reverted since it broke existing applications. Now with GTK+4 around the corner we can fix this. See the migration docs.
This will also allow us to get rid of all the rename-desktop-file in
the flatpak manifests too.
(The reason why this currently works in gnome-shell is because there's a private protocoll between GTK+ and GNOME Shell that (among other things) works around this).
Update: to check what app_id Wayland is seeing use:
WAYLAND_DEBUG=1 your_program |& grep 'xdg_toplevel@[0-9]\+\.set_app_id'
