This code is an alternate version of the code found in my tutorial on Nettuts+: http://net.tutsplus.com/tutorials/javascript-ajax/implement-twitter-scrolling-without-jquery-2

twitter-scrolling.js

$(document).bind("scroll", function (e) {
    scrolling = true;
});

setInterval(function () {
    if (scrolling) {
        //console.profile("scroll");
        scrolling = false;
        var temp_top = win.scrollTop(),
            down = scroll_top - temp_top < 0 ? true : false,
            top_item = null, child, style, i = 0, distance = 0;

        scroll_top = temp_top;
        if (prev_item) {
            height = height || prev_item.offsetHeight;
            if ( down && get_top_offset(prev_item) - scroll_top + height < margin) {
                top_item = entries[ entries.indexOf(prev_item) + 1]; 
            } else if ( !down && (get_top_offset(prev_item) - scroll_top - height) > (margin + parent_from_top)) {
                top_item = entries[ entries.indexOf(prev_item) - 1]; 
            } else {
                top_item = prev_item;
            }
        } else {
            for (i = 0; entries[i]; i++) {
                distance = get_top_offset(entries[i]) - scroll_top;
                if ( distance < (parent_from_top + margin) && distance > (parent_from_top - margin) ) {
                    top_item = entries[i];
                    break;
                }
            }
        }

        if (top_item) {
            if ( (down && top_item.getAttribute("data-8088-starter")) || ( !down && top_item.getAttribute("data-8088-ender") ) ) {
                if (!anti_repeater) {
                    child = unscrollable[ entries.indexOf(top_item) ];
                    anti_repeater = child.cloneNode(false);
                    document.body.appendChild(anti_repeater);
                    child.style.visibility = "hidden";
                    style = anti_repeater.style;
                    style.position = 'fixed';
                    style.top = img_offset.top + 'px';
                    style.left= img_offset.left + 'px';
                }
            } 
            if ( (down && top_item.getAttribute("data-8088-ender")) || (!down && top_item.getAttribute("data-8088-starter")) ) {
                unscrollable[ entries.indexOf(top_item) ].style.visibility = "visible";
                if (anti_repeater) { anti_repeater.parentNode.removeChild(anti_repeater); }
                anti_repeater = null;
            } 
        }
        prev_item = top_item;
        //console.profileEnd();
    }
}, 1);