Unable to view list of sessions on GDM screen - click on cog button in the right bottom corner does nothing

[Norbert]

Steps to reproduce:
1. Have Ubuntu 20.04 LTS installed with many desktop sessions
2. Boot the machine
3. Try to select graphical user session on the GDM greeter screen by mouse or by keyboard - by clicking on the cog button in the right bottom corner does nothing

Expected results:
* the list of graphical sessions is shown, may be scrolled by mouse and/or by keyboard

Actual results:
* the list of graphical sessions is not shown

----

$ ls /etc/X11/Xsession.d/
00deepin-dde-env 60x11-common_localhost
01gtk3-nocsd 60x11-common_xdg_path
20dbus_xdg-runtime 60xbrlapi
20desktop-profiles_activateDesktopProfiles 70gconfd_path-on-session
20flatpak 70gtk3-nocsd-propagate-LD_PRELOAD
20x11-common_process-args 70im-config_launch
30x11-common_xresources 70monkeysphere_use-validation-agent
35x11-common_xhost-local 70vlc-plugin-path
36x11-common_xhost-boinc 70xdg-kubuntu-dir
40x11-common_xsessionrc 75dbus_dbus-launch
50_check_unity_support 80kubuntu-xmodmap
50x11-common_determine-startup 80mate-environment
51gtk3-nocsd-detect 81overlay-scrollbar
52load-gtk-vector-screenshot-gtk-module 90atk-adaptor
55awesome-javaworkaround 90gpg-agent
55cinnamon-session_gnomerc 90qt-a11y
55gnome-session_gnomerc 90unclutter
55mate-session_materc 90x11-common_ssh-agent
55numlockx 95dbus_update-activation-env
55xfce4-session 99mate-environment
56ubuntustudio-session 99qt5ct
56xubuntu-session 99ukui-environment
60imwheel_start-imwheel 99x11-common_start

----

ProblemType: Bug
DistroRelease: Ubuntu Kylin 20.04
Package: gdm3 3.34.1-1ubuntu1 [origin: Ubuntu]
ProcVersionSignature: Ubuntu 5.4.0-28.32-lowlatency 5.4.30
Uname: Linux 5.4.0-28-lowlatency x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: X-Cinnamon
Date: Thu Apr 30 20:05:03 2020
InstallationDate: Installed on 2020-04-22 (7 days ago)
InstallationMedia: Ubuntu-MATE 20.04 LTS "Focal Fossa" - Release amd64 (20200422)
SourcePackage: gdm3
UpgradeStatus: No upgrade log present (probably fresh install)

[Sebastien Bacher (seb128)]

Thank you for your bug report. Could you add your 'journalctl -b 0' log to the bug after getting the issue? What sessions would you expect to see listed there (which one do you have installed?)
what's the output of 'ls -l /usr/share/gnome-session/sessions'

[Norbert (nrbrtx)]

```
$ ls -l /usr/share/gnome-session/sessions
total 64
-rw-r--r-- 1 root root 838 ноя 11 17:21 AfterStep.session
-rw-r--r-- 1 root root 2175 мар 21 18:10 budgie-desktop.session
-rw-r--r-- 1 root root 110 мая 25 2018 cairo-dock.session
-rw-r--r-- 1 root root 2672 апр 10 17:12 gnome-classic.session
-rw-r--r-- 1 root root 2012 мар 26 15:22 gnome-dummy.session
-rw-r--r-- 1 root root 664 мар 30 15:29 gnome-flashback-compiz.session
-rw-r--r-- 1 root root 668 мар 30 15:29 gnome-flashback-metacity.session
-rw-r--r-- 1 root root 712 июл 28 2019 gnome-flashback-xmonad.session
-rw-r--r-- 1 root root 621 мар 28 00:23 gnome-initial-setup.session
-rw-r--r-- 1 root root 562 окт 7 2019 gnome-login.session
-rw-r--r-- 1 root root 2171 мар 26 15:22 gnome.session
-rw-r--r-- 1 root root 692 мар 17 20:22 openbox-gnome-fallback.session
-rw-r--r-- 1 root root 724 мар 17 20:22 openbox-gnome.session
-rw-r--r-- 1 root root 614 апр 8 16:56 remmina-gnome.session
-rw-r--r-- 1 root root 588 мар 26 15:22 ubuntu.session
-rw-r--r-- 1 root root 101 мар 26 15:22 unity.session

```

[Norbert (nrbrtx)]

Part of the correct session list is shown on screenshot for lightdm with `ubuntu-mate-lightdm-theme` package.

[Sebastien Bacher (seb128)]

Could you edit /etc/gdm3/custom.conf , remove the # at the start of
#Enable=true
restart and add the journal log again?

[Norbert (nrbrtx)]

$ cat /etc/gdm3/custom.conf | grep Enable=true
Enable=true