Launch Probe.
Open Blank.fxml, Library, CV.
DnD a Button (pretty quickly) from Library to CV: nothing happen.
Try again: on get the exception:

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryListCell$2.handle(LibraryListCell.java:77)
at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryListCell$2.handle(LibraryListCell.java:62)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)




Now, relaunch Probe, and do the same actions but doing the DnD very slowly between Library and CV: it works well !
Then, try to move the Button in the CV: nothing happen (no exception).
Then, DnD a Checkbox from the Library to the CV, open the Hierarchy, and in the Hierarchy DnD the Button to put it below the Checkbox: it works well.
(I also tried to move nodes inside the Hierarchy with bigger scene graph, I never got any issue)



Doing other experiments (don't remembber how) I also got the exception:

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.mouse.SelectAndMoveGesture.mouseDragDetected(SelectAndMoveGesture.java:139)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.mouse.AbstractMouseDragGesture$1.handle(AbstractMouseDragGesture.java:57)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.content.gesture.mouse.AbstractMouseDragGesture$1.handle(AbstractMouseDragGesture.java:54)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)