Start the tool
Open css analyser panel using accelerator (cmd + 6)
Then, DND an item from the Library to the content view

You will get the following assertion error :
Glass detected outstanding Java exception at -[GlassViewDelegate sendJavaMouseEvent:]:src/com/sun/mat/ui/GlassViewDelegate.m:541
Exception in thread "AWT-AppKit" java.lang.RuntimeException: java.lang.AssertionError
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:251)
at com.sun.glass.ui.View.handleMouseEvent(View.java:528)
at com.sun.glass.ui.View.notifyMouse(View.java:922)
Caused by: java.lang.AssertionError
at com.oracle.javafx.authoring.stage.v2.EditModeController.enter(EditModeController.java:260)
at com.oracle.javafx.authoring.StageView.enter(StageView.java:686)
at com.oracle.javafx.authoring.controls.TreeDragContext.enterExit(TreeDragContext.java:256)
at com.oracle.javafx.authoring.controls.TreeDragContext.drag(TreeDragContext.java:277)
at com.oracle.javafx.authoring.panels.library.LibraryDragSupport.handleMouseDrag(LibraryDragSupport.java:67)
at com.oracle.javafx.authoring.panels.library.LibraryPanel$4.handle(LibraryPanel.java:128)
at com.oracle.javafx.authoring.panels.library.LibraryPanel$4.handle(LibraryPanel.java:123)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:64)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3324)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3164)
at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3119)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1559)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2261)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:228)
... 2 more

Note that if you do same scenario but using the menu item View -> Show CSS Analyser, you will not get the issue.