When doing:

$("#" + dataGrid).groupingGroupBy( groupColl ,{groupDataSorted : false, groupColumnShow: [false]});

The table shrinks a little bit. Afther a bit off debugging I stumbled on this code in grid.base.js:

if($t.p.shrinkToFit===true) {
      $($t).jqGrid("setGridWidth",$t.grid.width-0.001,true);
}

When I remove the -0.001 the shrinking problem is fixed. There is probably a reason why this -0.0001 was added but I don't know why but it definitely produces a problem in some situations.

I had a second problem when doing

$("#" + dataGrid).groupingRemove( true);

with these settings. I  had set to hide the grouping column when grouped but when you ungroup the column isn't shown again. I would expect the column to be visible again.

Great grid by the way