Gnome cannot start from systemd vnc: TypeError: this._userProxy.Display is null in getCurrentSessionProxy@resource:///org/gnome/shell/misc/loginManager.js

Affected version

Ubuntu 20.04 Gnome shell 3.36.3 Tigervnc 1.10.1

Bug summary

There seems to be a problem that people have been stumbling into now for many years with no resolution, and if you google it, you'll find frustrated people in far flung forums, mailing lists and support groups, namely that if you start a vnc session from systemd, Gnome won't start properly. Note that you can start a regular X session fine... you can start xterms and window managers etc, and from what I hear, you can start KDE fine as well.. but no so for Gnome. And you can start the vnc session from the command line and gnome starts fine. It's just when it comes from systemd it doesn't work. Based on my investigation, the most likely location of the issue is in Gnome, and most likely in gnome-shell.

Steps to reproduce

1. Install a vnc server. (I'm using Tiger VNC but I've seen people with other VNCs with the same issue)
2. set up a systemd specification in /etc/systemd/system/vncserver@.service (will give example below)
3. set up an instance of it (aka cd /etc/systemd/system ; ln -s vncserver@.service vncserver@1.service
4. start it up: sudo systemctl start vncserver@1
5. Connect to localhost:1 with a vnc client (Tiger VNC viewer is a good choice)

What happened

What you'll see is an Xsession without gnome. If you started other X programs, like an xterm, you'll see that, if not, you'll probably see a plain grey screen.

What did GNOME Shell do that was unexpected?

If you do a systemctl status vncserver@1.service

What you will see is:

Aug 03 23:50:44 ThinkPad gnome-shell[6467]: Unset XDG_SESSION_ID, getCurrentSessionProxy() called outside a user session. Asking logind directly.
Aug 03 23:50:44 ThinkPad gnome-shell[6467]: JS ERROR: TypeError: this._userProxy.Display is null
                                            getCurrentSessionProxy@resource:///org/gnome/shell/misc/loginManager.js:114:41
                                            ScreenShield@resource:///org/gnome/shell/ui/screenShield.js:105:28
                                            _initializeUI@resource:///org/gnome/shell/ui/main.js:200:24
                                            start@resource:///org/gnome/shell/ui/main.js:146:5
                                            @<main>:1:47
Aug 03 23:50:44 ThinkPad gnome-shell[6467]: Execution of main.js threw exception: Script <main> threw an exception
Aug 03 23:50:44 ThinkPad gnome-session[6047]: gnome-session-binary[6047]: WARNING: App 'org.gnome.Shell.desktop' exited with code 1
Aug 03 23:50:44 ThinkPad gnome-session[6047]: gnome-session-binary[6047]: WARNING: App 'org.gnome.Shell.desktop' respawning too quickly

From having a bit of a look around, this looks to be related to code in gnome-shell

What did you expect to happen

I expect to see a Gnome workspace, not a raw Xsession. Note that if I start a vncserver from a logged in command line, it works fine:

tigervncserver -localhost no :1

Relevant logs, screenshots, screencasts etc.

This is what I have in /etc/systemd/system/vncserver@.service

[Unit]
Description=Start TigerVNC server at startup
After=syslog.target network.target

[Service]
Type=simple
User=chris
#PAMName=login
Group=chris
WorkingDirectory=/home/chris

PIDFile=/home/chris/.vnc/%H:%i.pid
ExecStartPre=-/usr/bin/tigervncserver -kill :%i > /dev/null 2>&1
ExecStart=/usr/bin/tigervncserver -depth 24 -geometry 1280x800 -localhost no :%i
ExecStop=/usr/bin/tigervncserver -kill :%i

[Install]
WantedBy=multi-user.target

Note that the PAMName I have commented out, that was something I was experimenting with to see if it would help at all, but to no avail.

This is what I have in my xstartup:

#!/bin/bash
PATH=/usr/bin:/bin:$PATH

#unset SESSION_MANAGER
#unset DBUS_SESSION_BUS_ADDRESS

test -r $HOME/.Xresources && xrdb $HOME/.Xresources
vncconfig -iconic &
dbus-launch --exit-with-session gnome-session &
#exec /etc/X11/xinit/xinitrc
#gnome-session &
xterm

I tried various combinations of unsetting variables, starting different things. Basically X programs can start fine, I can start fvwm and xterm with no problem, but Gnome crashes.

Other things I tried was starting tigervncserver in the service with the runuser command.