Open the attached fxml.From the Content Panel, select the "SYNOPSIS" label.
From the Hierarchy, select all the nodes of the enclosing AnchorPane (i.e the HBox + its content, and the MenuButton)
Trying to Wrap in a new container, hit mouse right-click. The following Assertion is raised, and no popup is opened.

java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.editor.job.BatchModifyObjectJob.<init>(BatchModifyObjectJob.java:78)
at com.oracle.javafx.scenebuilder.kit.editor.job.UseComputedSizesObjectJob.removeAnchorsJobs(UseComputedSizesObjectJob.java:169)
at com.oracle.javafx.scenebuilder.kit.editor.job.UseComputedSizesObjectJob.buildSubJobs(UseComputedSizesObjectJob.java:134)
at com.oracle.javafx.scenebuilder.kit.editor.job.UseComputedSizesObjectJob.<init>(UseComputedSizesObjectJob.java:62)
at com.oracle.javafx.scenebuilder.kit.editor.job.UseComputedSizesSelectionJob.buildSubJobs(UseComputedSizesSelectionJob.java:151)
at com.oracle.javafx.scenebuilder.kit.editor.job.UseComputedSizesSelectionJob.<init>(UseComputedSizesSelectionJob.java:57)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.canPerformEditAction(EditorController.java:1262)
at com.oracle.javafx.scenebuilder.kit.editor.util.ContextMenuController$EditActionController.canPerform(ContextMenuController.java:505)
at com.oracle.javafx.scenebuilder.kit.editor.util.ContextMenuController.handleOnShowing(ContextMenuController.java:224)
at com.oracle.javafx.scenebuilder.kit.editor.util.ContextMenuController.access$000(ContextMenuController.java:60)
at com.oracle.javafx.scenebuilder.kit.editor.util.ContextMenuController$2.handle(ContextMenuController.java:124)
at com.oracle.javafx.scenebuilder.kit.editor.util.ContextMenuController$2.handle(ContextMenuController.java:119)
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.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:204)
at javafx.stage.Window.fireEvent(Window.java:1024)
at javafx.stage.Window$9.invalidated(Window.java:722)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:109)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:143)
at javafx.stage.Window.setShowing(Window.java:826)
at javafx.stage.Window.show(Window.java:841)
at javafx.stage.PopupWindow.showImpl(PopupWindow.java:446)
at javafx.stage.PopupWindow.show(PopupWindow.java:415)
at javafx.scene.control.ContextMenu.doShow(ContextMenu.java:286)
at javafx.scene.control.ContextMenu.show(ContextMenu.java:278)
at javafx.scene.control.Control$1.handle(Control.java:178)
at javafx.scene.control.Control$1.handle(Control.java:171)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
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.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.processMenuEvent(Scene.java:1735)
at javafx.scene.Scene.access$4500(Scene.java:201)
at javafx.scene.Scene$ScenePeerListener.menuEvent(Scene.java:2508)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$1.run(GlassViewEventHandler.java:361)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$1.run(GlassViewEventHandler.java:357)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMenuEvent(GlassViewEventHandler.java:357)
at com.sun.glass.ui.View.handleMenuEvent(View.java:534)
at com.sun.glass.ui.View.notifyMenu(View.java:850)
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)