code: function getLastVisibleItem() {
                          for (var i = lastIndexDisplayed; i >= firstIndexDisplayed; i--) {
                              if (that.items.itemAt(i)) {
                                  return that.items.itemAt(i);
                              }
                          }
                          return null;
                      }
docs: At a certain index, the VDS may return null for all items at that index and higher. When this is the case, the end marker should point to the last non-null item in the visible range.