Drop a Button.
Click on the root AnchorPane, from the Content Panel.
Press the mouse left on the Button + Drag:
a yellow area appears quickly, and the following assertion is raised:

java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.ContentPanelController.pick(ContentPanelController.java:419)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.ContentPanelController.pick(ContentPanelController.java:391)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture.dragOverGlassLayer(DragGesture.java:193)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture.dragEnteredGlassLayer(DragGesture.java:185)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture.start(DragGesture.java:156)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.activateGesture(EditModeController.java:853)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.dragEnteredGlassLayer(EditModeController.java:795)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.access$600(EditModeController.java:93)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController$7.handle(EditModeController.java:523)
at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController$7.handle(EditModeController.java:520)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:204)
at javafx.scene.Scene$DnDGesture.handleExitEnter(Scene.java:3152)
at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:3040)
at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2867)
at javafx.scene.Scene$DropTargetListener.dragEnter(Scene.java:2781)
at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$1.run(GlassSceneDnDEventHandler.java:59)
at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$1.run(GlassSceneDnDEventHandler.java:53)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragEnter(GlassSceneDnDEventHandler.java:53)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragEnter(GlassViewEventHandler.java:570)
at com.sun.glass.ui.View.handleDragEnter(View.java:659)
at com.sun.glass.ui.View.notifyDragEnter(View.java:981)
at com.sun.glass.ui.win.WinDnDClipboard.push(Native Method)
at com.sun.glass.ui.win.WinSystemClipboard.pushToSystem(WinSystemClipboard.java:234)
at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:51)
at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:59)
at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:274)
at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1149)
at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2911)
at javafx.scene.Scene$DnDGesture.process(Scene.java:2972)
at javafx.scene.Scene$DnDGesture.access$8400(Scene.java:2867)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3705)
at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3414)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1676)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2467)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:314)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:243)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:345)
at com.sun.glass.ui.View.handleMouseEvent(View.java:526)
at com.sun.glass.ui.View.notifyMouse(View.java:898)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$300(WinApplication.java:39)
at com.sun.glass.ui.win.WinApplication$4$1.run(WinApplication.java:112)
at java.lang.Thread.run(Thread.java:744)
Exception in thread "JavaFX Application Thread" E