src/Pure/GUI/gui.scala

 object GUI {
   /* Swing look-and-feel */
 
   def init_laf(): Unit = com.formdev.flatlaf.FlatLightLaf.setup()
 
-  def current_laf: String = UIManager.getLookAndFeel.getClass.getName()
+  def current_laf(): String = UIManager.getLookAndFeel.getClass.getName()
 
   def is_macos_laf: Boolean =
-    Platform.is_macos && UIManager.getSystemLookAndFeelClassName() == current_laf
+    Platform.is_macos && UIManager.getSystemLookAndFeelClassName() == current_laf()
 
   class Look_And_Feel(laf: LookAndFeel) extends Isabelle_System.Service {
     def info: UIManager.LookAndFeelInfo =
       new UIManager.LookAndFeelInfo(laf.getName, laf.getClass.getName)
   }