refactor(system): fix conflicts between desktop environment login managers

system/host/configuration.nix

@@ -116,7 +116,7 @@ in
 
       # Enable the KDE desktop environment.
       # https://kde.org/
-      kde.enable = false;
+      kde.useX11 = false;
 
       # Enable the XFCE desktop environment.
       # https://xfce.org/

system/modules/system/system.nix

@@ -18,6 +18,9 @@ in
     };
   };
   config = {
+    # Install the latest kernel
+    boot.kernelPackages = lib.mkDefault pkgs.linuxPackages_latest;
+
     # Set up the environment
     environment = {
       # Install base packages

system/modules/ui/desktops/budgie.nix

@@ -16,17 +16,18 @@ in
   };
 
   config = lib.mkIf cfg.enable {
-    aux.system.ui.desktops.enable = true;
+    aux.system.ui.desktops = {
+      enable = true;
+      displayManager = lib.mkOptionDefault "lightdm";
+    };
 
     services.xserver = {
       enable = true;
-      desktopManager.budgie.enable = true;
-      displayManager.lightdm.enable = lib.mkIf (
-        !(
-          config.services.xserver.displayManager.gdm.enable
-          || config.services.xserver.displayManager.sddm.enable
-        )
-      ) true;
+      desktopManager.budgie.enable =
+        if config.services.xserver.desktopManager.gnome.enable then
+          builtins.abort "Budgie and Gnome cannot be enabled at the same time due to a bug. For details and a possible workaround, please see https://discourse.nixos.org/t/help-i-cant-have-pantheon-gnome-and-plasma-installed-on-my-system-at-the-same-time/47346"
+        else
+          true;
     };
   };
 }

system/modules/ui/desktops/common.nix

@@ -21,6 +21,15 @@ in
           variant = "";
         };
       };
+      displayManager = lib.mkOption {
+        description = "The display manager to use to start a desktop session.";
+        type = lib.types.enum [
+          "gdm"
+          "lightdm"
+          "sddm"
+          "sddm_wayland"
+        ];
+      };
     };
   };
 
@@ -49,6 +58,17 @@ in
 
         # Configure keymap in X11
         xkb = config.aux.system.ui.desktops.xkb;
+
+        # Set the display manager
+        displayManager = {
+          gdm.enable = (cfg.displayManager == "gdm");
+          lightdm.enable = (cfg.displayManager == "lightdm");
+        };
+      };
+
+      displayManager = {
+        sddm.enable = (cfg.displayManager == "sddm");
+        sddm.wayland.enable = (cfg.displayManager == "sddm_wayland");
       };
 
       # Enable touchpad support (enabled by default in most desktop managers, buuuut just in case).

system/modules/ui/desktops/gnome.nix

@@ -16,15 +16,17 @@ in
   };
 
   config = lib.mkIf cfg.enable {
-    aux.system.ui.desktops.enable = true;
+    aux.system.ui.desktops = {
+      enable = true;
+      displayManager = lib.mkOverride 500 "gdm";
+    };
 
     # Enable Gnome
     services.xserver = {
       # Remove default packages that came with the install
       excludePackages = [ pkgs.xterm ];
 
-      desktopManager.gnome.enable = true;
-      displayManager.gdm.enable = true;
+      desktopManager.gnome.enable = lib.mkDefault true;
     };
 
     environment = {
@@ -44,6 +46,9 @@ in
       ];
     };
 
+    # Manually set askPassword to avoid a conflict with KDE
+    programs.ssh.askPassword = "${pkgs.gnome.seahorse}/libexec/seahorse/ssh-askpass";
+
     # Gnome UI integration for KDE apps
     qt = {
       enable = true;

system/modules/ui/desktops/kde.nix

@@ -18,7 +18,10 @@ in
   };
 
   config = lib.mkIf cfg.enable {
-    aux.system.ui.desktops.enable = true;
+    aux.system.ui.desktops = {
+      enable = true;
+      displayManager = if cfg.useX11 then lib.mkDefault "sddm" else lib.mkDefault "sddm_wayland";
+    };
 
     programs.dconf.enable = true;
 
@@ -30,18 +33,9 @@ in
     };
 
     services = {
-      displayManager.sddm.enable = true;
       desktopManager.plasma6.enable = true;
 
-      xserver.displayManager = lib.mkIf cfg.useX11 {
-        defaultSession = "plasmaX11";
-        sddm.wayland.enable = lib.mkIf (
-          !(
-            config.services.xserver.displayManager.gdm.enable
-            || config.services.xserver.displayManager.lightdm.enable
-          )
-        ) true;
-      };
+      xserver.displayManager = lib.mkIf cfg.useX11 { defaultSession = "plasmaX11"; };
     };
 
     # Enable Gnome integration

system/modules/ui/desktops/xfce.nix

@@ -16,15 +16,20 @@ in
   };
 
   config = lib.mkIf cfg.enable {
-    aux.system.ui.desktops.enable = true;
+    aux.system.ui.desktops = {
+      enable = true;
+      displayManager = lib.mkOptionDefault "lightdm";
+    };
 
-    services.xserver = {
+    services = {
+      displayManager.defaultSession = "xfce";
+      xserver = {
         enable = true;
         desktopManager = {
           xterm.enable = false;
           xfce.enable = true;
         };
-      displayManager.defaultSession = "xfce";
+      };
     };
   };
 }