code: function (rowDelta, colDelta) {
        if (priv.selEnd) {
          var td = self.view.getCellAtCoords({
            row: (priv.selEnd.row + rowDelta),
            col: priv.selEnd.col + colDelta
          });
          if (td) {
            selection.setRangeEnd(td);
          }
        }
      }
docs: Sets selection end cell relative to current selection end cell (if possible)