Find the offending element causing a horizontal scrollbar at any screen width (requires jQuery).

find-oversized-element.js

var screenWidth = window.innerWidth;
var visibleElements = jQuery(":visible");
visibleElements.each(function(){
    var $this = jQuery(this);
    if($this.width() > screenWidth){
        $this.css("border", "1px solid green");
        console.log("Screen width is " + screenWidth + " and the following element is " + $this.width(), $this);
    }
});