So it seems that the problem is not that xorg doesn't set DESKTOP_SESSION correctly, it's that SDDM uses an absolute path for this instead of just the base name like every other window manager.
I'll fix it in Ubuntu; meanwhile, a bug is filed upstream. Linked here. ** Also affects: sddm (Ubuntu) Importance: Undecided Status: New ** Changed in: xorg (Ubuntu Bionic) Status: Confirmed => Invalid ** Changed in: xorg (Ubuntu Bionic) Importance: High => Wishlist ** Changed in: xorg (Ubuntu Bionic) Assignee: Simon Quigley (tsimonq2) => (unassigned) ** Changed in: sddm (Ubuntu Bionic) Status: New => In Progress ** Changed in: sddm (Ubuntu Bionic) Importance: Undecided => High ** Changed in: sddm (Ubuntu Bionic) Assignee: (unassigned) => Simon Quigley (tsimonq2) ** Also affects: sddm via https://github.com/sddm/sddm/issues/852 Importance: Unknown Status: Unknown -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to xorg in Ubuntu. https://bugs.launchpad.net/bugs/1668722 Title: 60x11-common_xdg_path uses $DESKTOP_SESSION which needs a sanity check Status in SDDM: Unknown Status in sddm package in Ubuntu: In Progress Status in xorg package in Ubuntu: Invalid Status in sddm source package in Bionic: In Progress Status in xorg source package in Bionic: Invalid Bug description: This file: /etc/X11/Xsession.d/60x11-common_xdg_path contains references to $DESKTOP_SESSION variable. On one of my Ubuntu installs this variable contains: "plasma", on the other it contains: "/usr/share/xsessions/plasma". I don't know where either example comes from, but the upshot is that paths are added to XDG_DATA_DIRS and XDG_CONFIG_DIRS that don't exist. MANDATORY_PATH is also affected I believe, maybe others. Surely a sanity check of the generated paths is in order. Seems pointless to add non-existent ones. $XDG_SESSION_DESKTOP looks like a replacement for $DESKTOP_SESSION, but is that true? Cheers! To manage notifications about this bug go to: https://bugs.launchpad.net/sddm/+bug/1668722/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : firstname.lastname@example.org Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp