Couldn't figure out the actual reason but I did figure out a work-around.
{kind: "onyx.MenuDecorator", maxHeight: "5px", components: [
{name: "firstMonthContent", content: "Month"},
{name: "firstMonth", kind: "onyx.Menu", onSelect: "getFirstMonth", components: [
{name: "menuScroller2", kind: "enyo.Scroller", defaultKind: "onyx.MenuItem", vertical: "auto", classes: "enyo-unselectable", maxHeight: "200px", components: [
{value: "0", content: "Jan", classes: "menuItem"},
{value: "1", content: "Feb", classes: "menuItem"},
{value: "2", content: "Mar", classes: "menuItem"},
{value: "3", content: "Apr", classes: "menuItem"},
{value: "4", content: "May", classes: "menuItem"},
{value: "5", content: "Jun", classes: "menuItem"},
{value: "6", content: "Jul", classes: "menuItem"},
{value: "7", content: "Aug", classes: "menuItem"},
{value: "8", content: "Sep", classes: "menuItem"},
{value: "9", content: "Oct", classes: "menuItem"},
{value: "10", content: "Nov", classes: "menuItem"},
{value: "11", content: "Dec", classes: "menuItem"},
]}
]}
]},