jQuery UI menu inside a jqGrid cell

I have created a grid and customized a column to contain a jquery UI menu like in the Split Button example

Everything works fine except for the fact that the menu window appear inside the cell causing a bad visual effect, that is, the cell height increase to make room for the menu window. Have a look at the following screenshot for a visual explanation (nevermind about the menu item in disabled state).

Is there any way way to make the menu window appear on top of the table element in term of z-index?

Thanks very much for your valuable help, community :)

EDIT as per comment request:

The code to create the splitbutton menu is the following. First the column model markup

{ name: 'act', index: 'act', width: 80, sortable: false, search: false, align: 'center',
  formatter: function (cellvalue, options, rowObject) {
      var markup = "<div>" +
                      "<div class='actionsButtonset'>" +
                          "<button class='dshbd_ConfirmMonth' rel='" + rowObject.UmltID + "' rev='" + rowObject.IsConfirmAvailable + "' plock='" + rowObject.IsPeriodLocked + "' alt='Confirm'>Confirm</button>" +
                          "<button class='btnSelectMenu' rev='" + rowObject.IsUmltLocked + "' " + ">Select</button>" +
                      "</div>" +
                      "<ul class='actionMenu'>" +
                          "<li><a class='dshbd_UnlockMonth' href='#' rel='" + rowObject.UmltID + "' alt='Unlock'>Unlock</a></li>" +
                      "</ul>" +
                   "</div>";
      return markup;
  }
}

Then, inside the gridComplete event I have the following code (please note that some code is needed to enable/disable menu items

var confirmMonthBtn = $('.dshbd_ConfirmMonth');
$.each(confirmMonthBtn, function (key, value) {
    var button = $(this);
    var umltID = button.attr('rel');
    button.button().click(function (event) {
        event.preventDefault();
    });
    var isPeriodLocked = (button.attr('plock') === 'true');
    if (!isPeriodLocked) {
        var isConfirmAvailable = ($(this).attr('rev') === 'true');
        if (!isConfirmAvailable) {
            button.button({ disabled: true });
        }
    } else {
        button.button({ disabled: true });
    }
});
var currentPeriod = GetCurrentPeriod();
var period = GetCurrentViewPeriod();
var isCurrent = false;
if (currentPeriod != null && period != null) {
    isCurrent = period.PeriodID == currentPeriod.PeriodID;
}
var selectBtns = $('.btnSelectMenu');
$.each(selectBtns, function (key, value) {
    var button = $(this);
    button.button({ text: false, icons: { primary: 'ui-icon-triangle-1-s'} });
    button.click(function (event) {
        var menu = $(this).parent().next().show();
        menu.position({
            my: 'left top',
            at: 'left bottom',
            of: this
        });
        $(document).on('click', function () {
            menu.hide();
        });
        return false;
     });
     $('div.actionsButtonset').buttonset();
     var menuElement = button.parent().next();
     menuElement.hide();
     menuElement.menu({
         select: function (event, ui) {
             var umltID = ui.item.children().attr('rel');
             event.preventDefault();
         }
     });
     if (!isCurrent) {
         var isPeriodLocked = ($(this).attr('plock') === 'true');
         if (isPeriodLocked) {
             menuElement.menu({ disabled: false });
         } else {
             var isUmltLocked = ($(this).attr('rev') === 'true');
             menuElement.menu({ disabled: !isUmltLocked });
         }
     } else {
         //The current period is always unlocked
         menuElement.menu({ disabled: true });
     }
});

I prepared the demo for you which demonstrates how Split Button can be used inside of jqGrid. It displays

More detailed explanation of the demo I'll post later. Probably you will understand all yourself after examining of the code.