Position-fixed.js

   // Position fixed: pattern: https://github.com/shichuan/javascript-patterns/blob/master/jquery-patterns/window-scroll-event.html#L36
    // RequestAnimationFrame Shim
    window.animFrame = (function () {
      return window.requestAnimationFrame ||
      window.webkitRequestAnimationFrame ||
      window.mozRequestAnimationFrame ||
      function (callback) {
        window.setTimeout(callback, 15);
      };
    })();

    var scrollTimeout,
    $target = $('.target'),
    targetOffset = $target.offset();


    $(window).scroll(function () {
      if (scrollTimeout) {
        // clear the timeout, if one is pending
        // TODO: cerca come si cancella un requestAnimationFrame
        clearTimeout(scrollTimeout);
        scrollTimeout = null;
      }
      scrollTimeout = animFrame(scrollHandler, 15);
    });

    scrollHandler = function () {
      if ($(window).scrollTop() > targetOffset.top) {
       //do something with $target
      }
    };