diff -r 8d7029eb0c31 -r a3177973ca5e src/Pure/GUI/gui.scala
--- a/src/Pure/GUI/gui.scala	Tue Sep 24 19:41:14 2013 +0200
+++ b/src/Pure/GUI/gui.scala	Tue Sep 24 19:46:11 2013 +0200
@@ -35,6 +35,7 @@
   def init_laf(): Unit = UIManager.setLookAndFeel(get_laf())
 
   def is_macos_laf(): Boolean =
+    Platform.is_macos &&
     UIManager.getSystemLookAndFeelClassName() == UIManager.getLookAndFeel.getClass.getName