nixpkgs/pkgs/applications/display-managers/sddm/sddm-default-session.patch

diff --git a/src/common/Configuration.h b/src/common/Configuration.h
index 54bcace..49cf5cb 100644
--- a/src/common/Configuration.h
+++ b/src/common/Configuration.h
@@ -48,6 +48,8 @@ namespace SDDM {
         Entry(InputMethod,         QString,     QStringLiteral("qtvirtualkeyboard"),                   _S("Input method module"));
         Entry(Namespaces,          QStringList, QStringList(),                                  _S("Comma-separated list of Linux namespaces for user session to enter"));
         Entry(GreeterEnvironment,  QStringList, QStringList(),                                  _S("Comma-separated list of environment variables to be set"));
+        Entry(DefaultSession,      QString,     QString(),                                      _S("System-wide default session"));
+
         //  Name   Entries (but it's a regular class again)
         Section(Theme,
             Entry(ThemeDir,            QString,     _S(DATA_INSTALL_DIR "/themes"),             _S("Theme directory path"));
diff --git a/src/greeter/SessionModel.cpp b/src/greeter/SessionModel.cpp
index d8698b7..df3e3c4 100644
--- a/src/greeter/SessionModel.cpp
+++ b/src/greeter/SessionModel.cpp
@@ -49,6 +49,7 @@ namespace SDDM {
         if (dri_active)
             populate(Session::WaylandSession, mainConfig.Wayland.SessionDir.get());
         populate(Session::X11Session, mainConfig.X11.SessionDir.get());
+        selectDefaultSession();
         endResetModel();

         // refresh everytime a file is changed, added or removed
@@ -62,6 +63,7 @@ namespace SDDM {
             if (dri_active)
                 populate(Session::WaylandSession, mainConfig.Wayland.SessionDir.get());
             populate(Session::X11Session, mainConfig.X11.SessionDir.get());
+            selectDefaultSession();
             endResetModel();
         });
         watcher->addPaths(mainConfig.Wayland.SessionDir.get());
@@ -164,11 +166,25 @@ namespace SDDM {
                 delete si;
             }
         }
+    }
+
+    void SessionModel::selectDefaultSession() {
+        d->lastIndex = 0;
+
         // find out index of the last session
         for (int i = 0; i < d->sessions.size(); ++i) {
             if (d->sessions.at(i)->fileName() == stateConfig.Last.Session.get()) {
                 d->lastIndex = i;
-                break;
+                return;
+            }
+        }
+
+        // Otherwise, fallback to system-wide default session.
+        auto defaultSession = mainConfig.DefaultSession.get();
+        for (int i = 0; i < d->sessions.size(); ++i) {
+            if (QFileInfo(d->sessions.at(i)->fileName()).fileName() == defaultSession) {
+                d->lastIndex = i;
+                return;
             }
         }
     }
diff --git a/src/greeter/SessionModel.h b/src/greeter/SessionModel.h
index 8f4d539..02f77ce 100644
--- a/src/greeter/SessionModel.h
+++ b/src/greeter/SessionModel.h
@@ -59,6 +59,7 @@ namespace SDDM {
         SessionModelPrivate *d { nullptr };

         void populate(Session::Type type, const QStringList &dirPaths);
+        void selectDefaultSession();
     };
 }