This offset is used in many places, just search for `rowIndex` in the datatable source files and you'll find several instances.  I used it in `keynav.js` and in `editable.js`.  It *must* be calculated since it can change, for example, with nested headers.  It might even changed dynamically by the developer adding thead sections or rows to existing ones. If so, it might be better calculated on the fly like this:
```
rowIndexOffset = tbody.get('firstChild.rowIndex');
```