jqgrid subgrid scrollbar issue

HTML

<link rel="stylesheet" href="https://ajax.googleapis.com/ajax/libs/jqueryui/1.11.4/themes/redmond/jquery-ui.css">
<script src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.11.4/jquery-ui.min.js"></script>
<link rel="stylesheet" href="https://rawgit.com/tonytomov/jqGrid/master/css/ui.jqgrid.css">
<script src="https://rawgit.com/tonytomov/jqGrid/master/js/i18n/grid.locale-en.js"></script>
<script src="https://rawgit.com/tonytomov/jqGrid/master/js/jquery.jqGrid.js"></script>
<table id="jqGrid"></table>
<div id="jqGridPager"></div>

JavaScript

$.jgrid.extend({
  setColWidth: function(iCol, newWidth, adjustGridWidth) {
    "use strict";
    return this.each(function() {
      var $self = $(this),
        grid = this.grid,
        colName, colModel, i, nCol;
      if (typeof iCol === "string") {
        // the first parametrer is column name instead of index
        colName = iCol;
        colModel = $self.jqGrid("getGridParam", "colModel");
        for (i = 0, nCol = colModel.length; i < nCol; i++) {
          if (colModel[i].name === colName) {
            iCol = i;
            break;
          }
        }
        if (i >= nCol) {
          return; // error: non-existing column name specified as the first parameter
        }
      } else if (typeof iCol !== "number") {
        return; // error: wrong parameters
      }
      grid.resizing = {
        idx: iCol
      };
      grid.headers[iCol].newWidth = newWidth;
      if (adjustGridWidth !== false) {
        grid.newWidth = grid.width + newWidth - grid.headers[iCol].width;
      }
      grid.dragEnd(); // adjust column width
      if (adjustGridWidth !== false) {
        $self.jqGrid("setGridWidth", grid.newWidth, false); // adjust grid width too
      }
    });
  }
});

var mainGrid = {
  "total": 1,
  "page": 1,
  "pageSize": 20,
  "records": 1,
  "rows": [{
    "siteId": 12516086,
    "siteName": "Detroit",
    "siteAddress": "1 Woodward",
    "siteCitySt": "Detroit, MI",
    "siteZip": "48205",
    "productCd": "ProductX",
    "productName": "Product X",
    "finishedSize": null,
    "estimatedPieceWeight": 0.125,
    "vdpTypeCode": null,
    "taHouseholds": 1071689,
    "taDistribution": 445139,
    "distribution": 445139,
    "avgCpm": 32.32084084084084084084084084084084084084,
    "investment": 19542.8,
    "coverage": 41.54
  }]
};

var subGridData = {
  "total": 1,
  "page": 1,
  "pageSize": 20,
  "records": 2,
  "rows": [{
    "mediaPlanId": 3003,
    "mbuHdrId": 4035986,
    "mbuDtlId": 10442611,
    "commonMbuId": 99731,
    "ggId":...