I encountered the same issue in the same situation : iggrid is in a fixed table with width and height set to 100%, resulting in the fixed header not resizing properly.

Another bug was not notified in the previous posts : the width of the fixed header will be incorrect if there is a vertical scroll bar (it doesn't take into account the width of the scroll bar and will take 100% width (instead of 100% width – vscroll.width), resulting in an offset of the columns of the header and the body)

Here my code to workaround this until there is a proper fix by Infragistics.

• version 2013.2
• browsers : IE 8-9-10-11, Chrome 33

Also, how can i know when this bug is fixed by your dev team ?

Workaround code : 

function igGridWorkaroundResize(iggrid) {   //Initialize
  iggrid.igGrid({
    rendered: function (evt, ui) {
      setTimeout(function() { // setTimeout needed for IE8, other browsers are fine
        resizeIgGridBody();
        resizeIgGridHeader();
      }, 100);
    }
  });

  /* Force igGrid resizing when windows resize */
  $(window).resize(function () {
    resizeIgGridBody();
    resizeIgGridHeader();
  });

  function resizeIgGridBody() {
    // resize whole body to 100%
    iggrid.css("width", "100%");
    // hide horizontal scroller because it shouldn't appear
    $("#grid_hscroller_container").css("height", 0);
  }

  function resizeIgGridHeader() {
    // resize whole header to body width (because 100% doesn't take 'vertical scroll bar width' into account)
    var width = iggrid.width();
    iggrid.parent().parent().find(".ui-iggrid-headertable").css("width", width);
  }
}