Start the tool and import some node, let's say an anchor pane.
Move the mouse in content view area (if not already there).
Show the CSS panel using accelerators (cmd + 6), then hide it still using accelerators so the focus and the mouse stay in content view.
At this time, as soon as you move the mouse, you will get the following exception :

     [java] 2013-01-24 11:26:39.408 java[1707:707] [JRSAppKitAWT markAppIsDaemon]: Process manager already initialized: can't fully enable headless mode.
     [java] Glass detected outstanding Java exception at -[GlassViewDelegate sendJavaMouseEvent:]:src/com/sun/mat/ui/GlassViewDelegate.m:541
     [java] Exception in thread "AWT-AppKit" java.lang.RuntimeException: java.lang.AssertionError
     [java] at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:251)
     [java] at com.sun.glass.ui.View.handleMouseEvent(View.java:528)
     [java] at com.sun.glass.ui.View.notifyMouse(View.java:922)
     [java] Caused by: java.lang.AssertionError
     [java] at com.oracle.javafx.authoring.stage.v2.EditModeController.handleMouseEvent(EditModeController.java:187)
     [java] at com.oracle.javafx.authoring.stage.v2.EditModeController$1.handle(EditModeController.java:128)
     [java] at com.oracle.javafx.authoring.stage.v2.EditModeController$1.handle(EditModeController.java:125)
     [java] at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
     [java] at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
     [java] at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
     [java] at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
     [java] at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
     [java] at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
     [java] at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
     [java] at javafx.event.Event.fireEvent(Event.java:171)
     [java] at javafx.scene.Scene$MouseHandler.process(Scene.java:3324)
     [java] at javafx.scene.Scene$MouseHandler.process(Scene.java:3164)
     [java] at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3119)
     [java] at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1559)
     [java] at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2261)
     [java] at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:228)
     [java] ... 2 more