With new option multiboxonly i tried remove css style for multiselect checkboxes to get standard size. MSIE doesn't like that and extends height of header row. In this cell shouldn't be that span for column resizing and neither that div around it. It has no purpose in there because of resizable and sortable options to false.

And it would be fine, if cell with select all checkbox had it's own class like there area grid_sort and grid_resize, so it can be styled better.