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

diff --git a/src/common/Configuration.h b/src/common/Configuration.h
index cf44a62..7bb9c03 100644
--- a/src/common/Configuration.h
+++ b/src/common/Configuration.h
@@ -44,6 +44,7 @@ namespace SDDM {
                                                                                                    "NOTE: Currently ignored if autologin is enabled."));
         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(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 1953c76..54fe2f2 100644
--- a/src/greeter/SessionModel.cpp
+++ b/src/greeter/SessionModel.cpp
@@ -43,6 +43,7 @@ namespace SDDM {
         beginResetModel();
         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
@@ -52,6 +53,7 @@ namespace SDDM {
             d->sessions.clear();
             populate(Session::WaylandSession, mainConfig.Wayland.SessionDir.get());
             populate(Session::X11Session, mainConfig.X11.SessionDir.get());
+            selectDefaultSession();
             endResetModel();
         });
         watcher->addPath(mainConfig.Wayland.SessionDir.get());
@@ -149,11 +151,25 @@ namespace SDDM {
             else
                 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 2e2efa9..a93315c 100644
--- a/src/greeter/SessionModel.h
+++ b/src/greeter/SessionModel.h
@@ -58,6 +58,7 @@ namespace SDDM {
         SessionModelPrivate *d { nullptr };
 
         void populate(Session::Type type, const QString &path);
+        void selectDefaultSession();
     };
 }