### Question:
DragAndDropHelper { int getDelta(final MouseEvent event) { return (int) event.clientY - getClickedYPosition(); } DragAndDropHelper(final HTMLElement dragArea,
                             final HTMLElement addButtonContainer);   }### Answer:
@Test public void testGetDelta() { final int clickedYPosition = 20; final int mousePosition = 10; final int expected = mousePosition - clickedYPosition; final MouseEvent mouseEvent = mock(MouseEvent.class); mouseEvent.clientY = mousePosition; doReturn(clickedYPosition).when(helper).getClickedYPosition(); final int actual = helper.getDelta(mouseEvent); assertEquals(expected, actual); }