For some strange reason, since the last upgrade of my Fedora 23 box, I no longer can change screen brightness at all whenever I start a fresh GNOME session (I can still do it from GDM's screen, though), no matter it's for my regular user or for a different one (I've tried creating another user just to test, and did not work either).

The initial symptom was that using the special keys of my laptop would not work (not even the screen brightness overlay would show up), even though I could use the other special keys to change volume up and down. Then, after making sure that those keypresses were actually being recognized, I realized that not even changing the screen brightness from the top panel or gnome-control-center would work either.

After a bit of debugging, I found out that the problem is that the power plugin was early exiting here, in on_rr_screen_acquired():

    static void
    on_rr_screen_acquired (GObject      *object,
                           GAsyncResult *result,
                           gpointer      user_data)
    {
            GsdPowerManager *manager = user_data;
            GError *error = NULL;
    
            gnome_settings_profile_start (NULL);
    
            manager->priv->rr_screen = gnome_rr_screen_new_finish (result, &error);
    
            if (error) {
                    g_warning ("Could not create GnomeRRScreen: %s\n", error->message);
                    g_error_free (error);
                    gnome_settings_profile_end (NULL);
    
                    return;
            }
    
            /* set up the screens */
            [...]
    }

Looking at the commit where that early return was introduced[1], reading the bug description at [2] and hten double-checking that gnome-settings-daemon's PID is actually lower than gnome-shell's, makes me think this might actually be another incarnation of bug 719974, perhaps in gnome-shell even, but reporting it here for now. I'm adding Carlos Garnacho to CC, just in case.

My current workaround for this is simply de-activating and re-activating the power plugin again from org.gnome.settings-daemon.plugins.power.active GSetting property, or simply killing the g-s-d process and let it restart again, but it would be nice to fix this upstream.

Last, some relevant bits describiing my current environment:

 - Fedora 23 64 bit with Kernel 4.2.6-300.fc23.x86_64
 - gnome-session-xsession-3.18.1.2-2.fc23.x86_64
 - gnome-shell-3.18.1-1.fc23.x86_64
 - gnome-settings-daemon-3.18.1-1.fc23.x86_64

[1] https://mail.gnome.org/archives/commits-list/2013-December/msg01936.html
[2] https://bugzilla.gnome.org/show_bug.cgi?id=719974#c0