Doesn't happen all the time. I got it D&D a third ScrollBar.

Caused by: java.lang.AssertionError: 10 calls to layout() were not enough to layout class javafx.scene.layout.Pane
at com.oracle.javafx.authoring.util.Utils.forceMultipleLayout(Utils.java:581)
at com.oracle.javafx.authoring.stage.v2.DropOperation.run(DropOperation.java:68)
at com.oracle.javafx.authoring.stage.v2.NewComponentDndGesture.drop(NewComponentDndGesture.java:78)
at com.oracle.javafx.authoring.stage.v2.EditModeController.drop(EditModeController.java:291)
at com.oracle.javafx.authoring.StageView.drop(StageView.java:665)
at com.oracle.javafx.authoring.controls.TreeDragContext.finish(TreeDragContext.java:187)
at com.oracle.javafx.authoring.controls.TreeDragContext.finish(TreeDragContext.java:169)
at com.oracle.javafx.authoring.panels.library.LibraryDragSupport.handleMouseRelease(LibraryDragSupport.java:73)
at com.oracle.javafx.authoring.panels.library.LibraryPanel$2.handle(LibraryPanel.java:102)
at com.oracle.javafx.authoring.panels.library.LibraryPanel$2.handle(LibraryPanel.java:98)
at com.sun.javafx.event.CompositeEventHandler.dispatchCapturingEvent(CompositeEventHandler.java:78)
at com.sun.javafx.event.EventHandlerManager.dispatchCapturingEvent(EventHandlerManager.java:202)
at com.sun.javafx.event.EventHandlerManager.dispatchCapturingEvent(EventHandlerManager.java:159)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchCapturingEvent(CompositeEventDispatcher.java:22)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:31)
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:3319)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3159)
at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3114)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1536)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2256)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:227)
... 2 more