Today I updated jqgrid from 3.8.something to 4.1.2 and I found the same issue. I think I found the BUG cause.

It seems that the autowidth calculates the width BEFORE hiding the group column.

In fact if I remove the groupColumnShow : [false] the table returns to 100%, if I keep the option the width is less then 100%

It seems that the autowidth problem occurs even when hidden columns are used.

In this case if I hide all columns except one, the table width will have the same widthas the case when I decide to show all columns.

The table width is calculated before hiding the columns so the final result is a overlarge table

BTW any ETA on the autowidth bug fix?

I just found the solution :

We only need to set “hidden: true” on the grouped column in colModel