nixos/x11/display-managers: don't touch graphical-session.target in xsession-wrapper if the desktop knows how to handle it

This is not correct and will in fact break things because they try to run before the target is reached.
Ideally we'd get rid of it entirely, but WM users rely on this behavior, so allowlist some desktops
to get the sane behavior, and fake the session for the rest until upstreams/NixOS modules catch up.

nixos/modules/services/x11/display-managers/default.nix

@@ -27,6 +27,30 @@ let
     Xft.hintstyle: ${fontconfig.hinting.style}
   '';
 
+  # FIXME: this is an ugly hack.
+  # Some sessions (read: most WMs) don't activate systemd's `graphical-session.target`.
+  # Other sessions (read: most non-WMs) expect `graphical-session.target` to be reached
+  # when the entire session is actually ready. We used to just unconditionally force
+  # `graphical-session.target` to be activated in the session wrapper so things like
+  # xdg-autostart-generator work on sessions that are wrong, but this broke sessions
+  # that do things right. So, preserve this behavior (with some extra steps) by matching
+  # on XDG_CURRENT_DESKTOP and deliberately ignoring sessions we know can do the right thing.
+  fakeSession = action: ''
+      session_is_systemd_aware=$(
+        IFS=:
+        for i in $XDG_CURRENT_DESKTOP; do
+          case $i in
+            KDE|GNOME|X-NIXOS-SYSTEMD-AWARE) echo "1"; exit; ;;
+            *) ;;
+          esac
+        done
+      )
+
+      if [ -z "$session_is_systemd_aware" ]; then
+        /run/current-system/systemd/bin/systemctl --user ${action} nixos-fake-graphical-session.target
+      fi
+  '';
+
   # file provided by services.xserver.displayManager.sessionData.wrapper
   xsessionWrapper = pkgs.writeScript "xsession-wrapper"
     ''
@@ -90,8 +114,7 @@ let
 
       ${cfg.displayManager.sessionCommands}
 
-      # Start systemd user services for graphical sessions
-      /run/current-system/systemd/bin/systemctl --user start graphical-session.target
+      ${fakeSession "start"}
 
       # Allow the user to setup a custom session type.
       if test -x ~/.xsession; then
@@ -417,10 +440,10 @@ in
       "XDG_SESSION_ID"
     ];
 
-    systemd.user.targets.graphical-session = {
+    systemd.user.targets.nixos-fake-graphical-session = {
       unitConfig = {
-        RefuseManualStart = false;
-        StopWhenUnneeded = false;
+        Description = "Fake graphical-session target for non-systemd-aware sessions";
+        BindsTo = "graphical-session.target";
       };
     };
 
@@ -451,7 +474,7 @@ in
 
           test -n "$waitPID" && wait "$waitPID"
 
-          /run/current-system/systemd/bin/systemctl --user stop graphical-session.target
+          ${fakeSession "stop"}
 
           exit 0
         '';