Gets or sets the range anchor index.
IndexPath RangeAnchorIndex { get; set; }
The range anchor index holds the index of the item from which ranged selection will start, i.e. when shift-clicking an item it represents the start of the range to be selected whereas the shift-clicked item will be the end of the range.
RangeAnchorIndex is set when selecting an item via SelectedIndex but not via Select(IndexPath).