Just trying the Cell editing feature in Demo, Steel theme, and I see a weird behaviour about the datepicker.

If the scroll of the grid is zero (eg. in initial state) the datepicker display in the right position; when I scroll down to the last row of the first page of the grid, and open it, it opens far outside the grid. It seems it doesn't subtract the grid body hidden part to the y coordinate where to show.

I tend to think it's a datepicker problem, not jqGrid.

Maybe it worth further investigations …