### Question:
DragAndDropHelper { Object onDragAreaMouseUp(final Event event) { draggingElement = null; clickedYPosition = 0; startYPosition = 0; refreshItemsPosition(); return this; } DragAndDropHelper(final HTMLElement dragArea,
                             final HTMLElement addButtonContainer);   }### Answer:
@Test public void testOnDragAreaMouseUp() { final Event event = mock(Event.class); doNothing().when(helper).refreshItemsPosition(); helper.onDragAreaMouseUp(event); assertEquals(null, helper.getDragging()); assertEquals(0, helper.getClickedYPosition()); assertEquals(0, helper.getStartYPosition()); verify(helper).refreshItemsPosition(); }