--- a/testing/marionette/harness/marionette/tests/unit/test_set_window_size.py
+++ b/testing/marionette/harness/marionette/tests/unit/test_set_window_size.py
@@ -46,16 +46,18 @@ class TestSetWindowSize(MarionetteTestCa
     def test_possible_to_request_window_larger_than_screen(self):
         self.marionette.set_window_size(100000, 100000)
         size = self.marionette.window_size
 
         # In X the window size may be greater than the bounds of the screen
         self.assertGreaterEqual(size["width"], self.max_width)
         self.assertGreaterEqual(size["height"], self.max_height)
 
+    # Note that this test will only pass in X
+    # when run in a window manager
     def test_that_we_can_maximise_the_window(self):
         # valid size
         width = self.max_width - 100
         height = self.max_height - 100
         self.marionette.set_window_size(width, height)
 
         # event handler
         self.marionette.execute_script("""