diff --git a/scanslicer/main.py b/scanslicer/main.py
index 39a6e43..683ed71 100644
--- a/scanslicer/main.py
+++ b/scanslicer/main.py
@@ -443,3 +443,21 @@ class QtScanSlicer(QMainWindow, ScanSlicerUiBuilderMixin, ScanSlicerControllerMi
         if hasattr(self, "_close_status_log"):
             self._close_status_log()
         super().closeEvent(event)
+
+# run_app: function definition for ScanSlicer behavior.
+def run_app(startup_dir=None):
+    app = QApplication.instance()
+    if app is None:
+        app = QApplication(sys.argv)
+    apply_dark_theme_if_requested(app)
+    apply_global_text_size_offset(app, 2)
+    app.setApplicationName("ScanSlicer")
+    app.setApplicationDisplayName("ScanSlicer")
+    app.setDesktopFileName("scanslicer")
+    app_icon = resolve_app_icon()
+    if not app_icon.isNull():
+        app.setWindowIcon(app_icon)
+
+    scan_slicer_window = QtScanSlicer(startup_dir)
+    scan_slicer_window.showMaximized()
+    return app.exec()