Feature Detection To Replace Use Of "navigator.userAgent" In Yahoo! UI Event.js?

I'm picking apart the Yahoo! UI event.js library and stripping it down
to just what I need. I'm also trying to make the parts I use better.
I'm stumped on how to fix the code for the getPageX() method. This
method is supposed to give the same value in IE as other browsers for
the event position relative to the left of the page. This value will be
greater than the position relative to the left of the browser if the
browser is scrolled to the right. What would be the appropriate feature
detection to use for the conditional in the following line?

if ( this.isIE ) {

Thank you,
Peter


isSafari: (/Safari|Konqueror|KHTML/gi).test(navigator.userAgent),

isIE: (!this.isSafari && !navigator.userAgent.match(/opera/gi) &&
navigator.userAgent.match(/msie/gi)),


getPageX: function(ev) {
var x = ev.pageX;
if (!x && 0 !== x) {
x = ev.clientX || 0;

if ( this.isIE ) {
x += this._getScroll()[1];
}
}
return x;
},

_getScroll: function() {
var dd = document.documentElement, db = document.body;
if (dd && dd.scrollTop) {
return [dd.scrollTop, dd.scrollLeft];
} else if (db) {
return [db.scrollTop, db.scrollLeft];
}
return [0, 0];
}