This is a technical document detailing exactly how Lubuntu does XDG settings, some problems we have faced, and anything else worth mentioning.
Here's a step-by-step on what pulls from what:
- SDDM pulls the session name from /usr/share/xsessions when logging in. In our case, it's Lubuntu.desktop. Therefore, the DESKTOP_SESSION environment variable is set to Lubuntu, from this filename. Please note that we had to patch SDDM to do this properly; it used to make DESKTOP_SESSION an absolute path, which, when we continue in these steps, breaks things. TODO: send this upstream.
- xorg sets the XDG_CONFIG_DIRS and XDG_DATA_DIRS environment variables to /etc/xdg/xdg-DESKTOP_SESSION and /etc/xdg at minimum. NOTE: this is an Ubuntu-specific patch in xorg.
- startlxqt then grabs the values of XDG_CONFIG_DIRS and XDG_DATA_DIRS to use in the call to startlxqt, which then does a one-time copy of the settings from the first entry in both.
For a while before we could figure it out, the Lubuntu Next 18.04 image had a black screen on the live CD and on bootup of a new Lubuntu Next system. The LXQt system had been logged in, but nothing was visible. This is why we had to patch SDDM, because the value of XDG_CONFIG_DIRS didn't have our valid XDG path; it was an absolute path put onto /etc/xdg/xdg-. So therefore, if the situation ever comes up again, it means that XDG_CONFIG_DIRS can't find valid settings. This also caused Simon to confront upstream; they had moved the default settings to /usr/share from /etc/xdg because they believed that it would apparently make it easier for distributors. Not only did they not consider that /usr/share is not in the XDG spec, they did no verification in LXQt itself to ensure that /usr/share at minimum is always there. When Simon submitted a patch, it was rejected on the grounds that it was too distro-specific, which was ironic because they didn't follow the XDG spec. Alf has since reconsidered and asked Simon to resubmit the patch.
If you rename /usr/share/xsessions/Lubuntu.desktop to something else, please do change /etc/xdg/xdg-Lubuntu/ to the same name. Otherwise none of our settings will be applied.