doc of Event.copyFor:

"Creates and returns a copy of this event with the specified event source and target."

formulated as test (can be run in EventAnyTest):

    @Test
    public void testCopyFor() {
        Object sourceForCopy = new Object();
        Event copiedEvent = event.copyFor(sourceForCopy, event.getTarget());
        assertEquals("copied " + event.getClass().getName() + " must have copied source\n",
                sourceForCopy, copiedEvent.getSource());
    }

- fails for TreeItem.TreeModificationEvent, all EditEvents (TreeTable, Tree, List)
- passes for CheckBoxTreeItem.TreeModificationEvent, ScrollToEvent, SortEvent, Tree/TableColumn.CellEditEvent

Culprit is a shadowing field source which is returned in the overridden getSource, f.i for ListView.EditEvent:

    final ListView<T> source;

    public EditEvent(ListView<T> source, ..)
         super(source, ..)
         this.source = source;
         ...
     }

     @Override public ListView<T> getSource() {
            return source;
      }
  
fix would be to remove the new field and the override. This will break client code relying on the incorrect source being a ListView always - but such code would be plain wrong anyway.