scrollable.js

var interval;

// after the complete ribbon is loaded, "listen" to the
// "Insert Merge Field" drop-down to see whether the
// database is loaded completely
TXTextControl.addEventListener("ribbonTabsLoaded", function (e) {
    interval = setInterval(SetScrollbars, 500);
});

// if button doesn't have the "disabled" class anymore
function SetScrollbars() {
    if (!$("#drpDnBtnInsertMergeField").hasClass("ui-state-disabled")) {

        clearInterval(interval);

        // find the last "ul" element and set the height and
        // overflow properties
        $("#insertMergeFieldDropDown").find("li").each(function () {
            $(this).find("ul").last().css({
                "max-height": "150px",
                "overflow-y": "scroll",
                "width": $(this).width() + 50
            });
        });
    }
};