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  //add in our z-indexstuff
  var index_highest = 0;
  // Search all elements with '*'
  $("*").each(function() {
      var index_current = parseInt($(this).css("zIndex"), 10);
      if (index_current > index_highest) {
          index_highest = index_current;
      }
  });

  //now the highest number is obtained and captured in the 'index_highest' variable - add 1 to it and assign to your element
  $('.your-element-to-be-the-highest-z-index').css({
    'z-index':index_highest+1
  });
  
  //ref - http://stackoverflow.com/questions/5680770/how-to-find-the-highest-z-index-using-jquery