sddm will not start lxqt desktop correctly

ok, it's a bit superfluous:

apt-get install xorg lxqt sddm xfwm4

is really

apt install lxqt xinit - to be more verbose;

sddm is the recommended dm
sddm-maui is the recommended theme
xfwm4 is the recommended WM
the very most needed X-related packages should be thrown in by default, except xinit - the dependencies of xorg are a bit overkill

Without further modifications sddm will start, lxqt will start - everything is fine, except the xdg-config-path which is hard set in /etc/X11/Xsession.d/60x11-common_xdg_path - it should fix that. (i guess, that deleting that file is not an option :) )

so - whats happend: without deleting or extending 60x11-common_xdg_path the minimum upstream configurations are not found, no configuration package was loaded so LXQt will start without any configuration. And it start perfectly fine - ok, no wm is found, the design is black in black, the black panel on a black background has no widgets - but it starts fine.

Conclusion:
Option 1: Somewhere before 99x11-common_start XDG_CONFIG_DIRS should be extended by /usr/share
Option 2: the session should depend on or recommend a settings package which put settings into /etc/xdg
Option 3: the lxqt-session itself should be patched to include /usr/share - like the current state in cosmic.

I also have a Debian Stretch laptop with LXQT installed on it.
I notice that this does have an /etc/xdg/lxqt folder
This appears to be created by Debian package lxqt-common

On Ubuntu Bionic if I try to install lxqt-common I get:-

root@t470:~# apt-get install lxqt-common
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
  ffmpegthumbnailer gvfs-fuse libffmpegthumbnailer4v5 liblxqt-globalkeys-ui0 liblxqt-globalkeys0 libmuparser2v5 libstatgrab10 libsysstat-qt5-0
  libupower-glib3 lxqt-globalkeys-l10n lxqt-notificationd-l10n lxqt-panel-l10n lxqt-policykit-l10n lxqt-powermanagement-l10n lxqt-runner-l10n
  lxqt-session-l10n pcmanfm-qt-l10n upower
Use 'apt autoremove' to remove them.
The following additional packages will be installed:
  breeze-cursor-theme fonts-noto fonts-noto-cjk fonts-noto-hinted fonts-noto-unhinted policykit-1-gnome xfwm4-theme-breeze
Suggested packages:
  fonts-noto-cjk-extra
The following packages will be REMOVED:
  lxqt lxqt-core lxqt-globalkeys lxqt-notificationd lxqt-panel lxqt-policykit lxqt-powermanagement lxqt-runner lxqt-session lxqt-system-theme
  lxqt-themes pcmanfm-qt qlipper
The following NEW packages will be installed:
  breeze-cursor-theme fonts-noto fonts-noto-cjk fonts-noto-hinted fonts-noto-unhinted lxqt-common policykit-1-gnome xfwm4-theme-breeze
0 upgraded, 8 newly installed, 13 to remove and 1 not upgraded.
Need to get 65.7 MB of archives.
After this operation, 121 MB of additional disk space will be used.
Do you want to continue? [Y/n]

I'm sorry but I don't have the skills to figure out why installation of lxqt-common causes uninstallation of lxqt

On the Ubuntu laptop:-

root@t470:/# ls -al /etc/xdg/lxqt/
total 12
drwxr-xr-x 2 root root 4096 Jun 18 00:37 .
drwxr-xr-x 10 root root 4096 Jun 18 00:37 ..
-rw-r--r-- 1 root root 657 Oct 21 2017 globalkeyshortcuts.conf

on the Debian one I also have:-
lxqt.conf
panel.conf
session.conf
windowmanagers.conf

lxqt-common is deprecated in upstream, debian and ubuntu - and anywhere else. It will be removed from debian testing soon and from sid after the upcoming debian release - please see also the respective launchpad bug in lxqt-common. In short: lxqt is not co-installable with the current version of LXQt.

All files that was former in lxqt-common are moved to the packages where they belong to. If you run a pure ubuntu installation with LXQt as only desktop environment you have some sane choices:
* fix 60x11-common-xdg... (just add /usr/share to the config_dirs) or delete this file - or maybe better copy that file to 61x11.. and leave only the fixed path in
* update right now to cosmic
* backport the patch from cosmic to your package

ok thanks
What I just tried
clean install of ubuntu server
updgrade to cosmic
install lxqt

root@t470:~# history
    1 apt-get update
    2 apt-get upgrade
    3 apt autoremove
    4 reboot
    5 vi /etc/update-manager/release-upgrades
    6 do-release-upgrade -d
    7 reboot
    8 apt-get update
    9 apt-get upgrade
   10 apt-get install lxqt xinit
   11 reboot

It works beautifully.

Thank you.

When the config files moved from /etc/xdg/lxqt to /usr/share/lxqt
/usr/bin/startlxqt should have had

if [ -z "$XDG_CONFIG_DIRS" ]; then
    export XDG_CONFIG_DIRS="/etc:/etc/xdg:/usr/share"
else
    if ! contains "$XDG_CONFIG_DIRS" '/etc/xdg'; then
        XDG_CONFIG_DIRS="$XDG_CONFIG_DIRS:/etc/xdg"
    fi
fi

modified to

if [ -z "$XDG_CONFIG_DIRS" ]; then
    export XDG_CONFIG_DIRS="/etc:/etc/xdg:/usr/share"
else
    if ! contains "$XDG_CONFIG_DIRS" '/etc/xdg'; then
        XDG_CONFIG_DIRS="$XDG_CONFIG_DIRS:/etc/xdg"
    fi
    if ! contains "$XDG_CONFIG_DIRS" '/usr/share'; then
        XDG_CONFIG_DIRS="$XDG_CONFIG_DIRS:/usr/share"
    fi
fi

*OR* placed the files in /etc/xdg/lxqt like lxqt-globalkeys does

*OR* provided a convenience symlink compatible with x11-common's 60x11-common_xdg_path of
/etc/xdg/xdg-lxqt/lxqt -> /usr/share/lxqt

What I'm hearing is this has been fixed since at least Cosmic.