Update:
in the onSortColumn
I used:
var numpages= parseInt(o.datagrid('getData').total / o.datagrid('options').pageSize) +1;
var index= -1;
for(var i=0; i<= numpages && index ==-1; i++)
{
o.datagrid('gotoPage',i);
index= o.datagrid('getRowIndex',myId) ;
}
But if there are more than 1 page, index is always 499 (pageSize is 500)
so I think there's a bug