Is the document scrolling right now? Prevent jank by pausing stuff while the user's trying to navigate the document.

isScrolling.js

var isScrolling = false;
$(document).ready(function(){
    var scrollTimeout = false;
    var scrollTimeoutFn = function(){
        isScrolling=false;
        scrollTimeout = false;
    }
    jQuery(window).bind('scroll', function(){
        isScrolling = true;
        if(scrollTimeout){
            window.clearTimeout(scrollTimeout);
        }
        scrollTimeout = window.setTimeout(scrollTimeoutFn,300);
    });
});