The problem seems to be that, when the browser scroll-bar is present, the initial load of the grid results in the grid scroll-bar sitting outside of the grid parent element. My grids all sit inside a parent DIV which provides an external title bar and external buttons to the grid.

See the video for a demonstration in Chrome (I have reproduced the problem in IE8, Firefox and Chrome).
http://screencast.com/t/ZjdkYjQ4

I'm Using jqGrid 3.6.5

The video begins showing a grid after initial load with few grid items (no grid scroll-bar), no browser scroll-bar, autoWidth:true and shrinkToFit:true

@ 10 seconds: I resize the browser to introduce its vertical scroll bar
@ 20 seconds: I demonstrate my custom window.resize setGridWidth() tracking