kde: force dolphin as file manager

system/kde.nix

@@ -22,5 +22,20 @@
       kdePackages.partitionmanager
       kdePackages.sddm-kcm
     ];
+
+    xdg.portal = {
+      enable = true;
+      extraPortals = [ pkgs.kdePackages.xdg-desktop-portal-kde ];
+      config = {
+        common = {
+          default = [ "kde" ];
+        };
+      };
+    };
+
+    environment.sessionVariables = {
+      GTK_USE_PORTAL = "1";
+      GDK_DEBUG = "portals";
+    };
   };
 }

user/kde.nix

@@ -9,4 +9,9 @@
 }
 // lib.mkIf (config.my.desktop == "kde") {
   services.gpg-agent.pinentry.package = lib.mkForce pkgs.pinentry-qt;
+
+  # Some programs like Firefox and vscode don't use xdg-open, and instead use org.freedesktop.FileManager1.
+  # This forces Dolphin to be the provider for that.
+  xdg.dataFile."dbus-1/services/org.freedesktop.FileManager1.service".source =
+    "${pkgs.kdePackages.dolphin}/share/dbus-1/services/org.kde.dolphin.FileManager1.service";
 }