This occurs with Group, MeshView, AmbientLight, ParallelCamera, PerspectiveCamera, PointLight, etc ... When occurring you've no choice but restart SceneBuilder.

Exception in thread "JavaFX Application Thread" java.lang.IllegalArgumentException: Both width and height must be >= 0
    at javafx.geometry.Rectangle2D.<init>(Rectangle2D.java:104)
    at com.oracle.javafx.scenebuilder.kit.editor.drag.source.DragSourceShadow.setupForNode(DragSourceShadow.java:86)
    at com.oracle.javafx.scenebuilder.kit.editor.drag.source.LibraryDragSource.makeShadow(LibraryDragSource.java:199)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture.showShadow(DragGesture.java:370)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture.dragEnteredGlassLayer(DragGesture.java:182)
    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:872)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController.dragEnteredGlassLayer(EditModeController.java:810)
    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:538)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.mode.EditModeController$7.handle(EditModeController.java:535)
    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:3197)
    at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:3085)
    at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2896)
    at javafx.scene.Scene$DropTargetListener.dragOver(Scene.java:2817)
    at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:106)
    at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:102)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragOver(GlassSceneDnDEventHandler.java:102)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragOver(GlassViewEventHandler.java:598)
    at com.sun.glass.ui.View.handleDragOver(View.java:668)
    at com.sun.glass.ui.View.notifyDragOver(View.java:986)
    at com.sun.glass.ui.mac.MacPasteboard._putItemsFromArray(Native Method)
    at com.sun.glass.ui.mac.MacPasteboard.putItemsFromArray(MacPasteboard.java:143)
    at com.sun.glass.ui.mac.MacPasteboard.putItems(MacPasteboard.java:165)
    at com.sun.glass.ui.mac.MacSystemClipboard.pushToSystem(MacSystemClipboard.java:223)
    at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:51)
    at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:59)
    at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:274)
    at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1150)
    at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2940)
    at javafx.scene.Scene$DnDGesture.process(Scene.java:3009)
    at javafx.scene.Scene$DnDGesture.access$8400(Scene.java:2896)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3762)
    at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3471)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1695)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2486)
    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)
Exception in thread "JavaFX Application Thread" java.lang.AssertionError
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture.updateShadow(DragGesture.java:378)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture.dragOverGlassLayer(DragGesture.java:263)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture.access$100(DragGesture.java:65)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture$1.handle(DragGesture.java:107)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.DragGesture$1.handle(DragGesture.java:103)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.event.Event.fireEvent(Event.java:204)
    at javafx.scene.Scene$DnDGesture.fireEvent(Scene.java:2920)
    at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:3092)
    at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2896)
    at javafx.scene.Scene$DropTargetListener.dragOver(Scene.java:2817)
    at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:106)
    at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:102)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragOver(GlassSceneDnDEventHandler.java:102)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragOver(GlassViewEventHandler.java:598)
    at com.sun.glass.ui.View.handleDragOver(View.java:668)
    at com.sun.glass.ui.View.notifyDragOver(View.java:986)
    at com.sun.glass.ui.mac.MacPasteboard._putItemsFromArray(Native Method)
    at com.sun.glass.ui.mac.MacPasteboard.putItemsFromArray(MacPasteboard.java:143)
    at com.sun.glass.ui.mac.MacPasteboard.putItems(MacPasteboard.java:165)
    at com.sun.glass.ui.mac.MacSystemClipboard.pushToSystem(MacSystemClipboard.java:223)
    at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:51)
    at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:59)
    at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:274)
    at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1150)
    at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2940)
    at javafx.scene.Scene$DnDGesture.process(Scene.java:3009)
    at javafx.scene.Scene$DnDGesture.access$8400(Scene.java:2896)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3762)
    at javafx.scene.Scene$MouseHandler.access$1800(Scene.java:3471)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1695)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2486)
    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)