滚动跟踪，简单的实现，range控制滚动的上下限（和range指定的元素上下边界对齐），支持超出一屏高度的调整

scrollFollow.coffee

  scrollFollow = (ele, opts)=>
    ele = $ ele
    range = $ opts.range
    n = 0
    lastTop = $(window).scrollTop() - 1
    scrolling = ->
      top = $(window).scrollTop()
      viewHeight = $(window).height()
      eleHeight = origEleHeight = ele.height()
      eleHeight = viewHeight if eleHeight < viewHeight
      
      if top < rangeTop = range.offset().top
        reset = ->
          ele.css
            'position': ''
            'display': ''
            'top': ''
        if ele.css('position') is 'absolute'
          ele.stop(yes).animate
            'top': rangeTop
            'fast'
            reset
        else
          reset()

      else
        if top + viewHeight > (rangeBottom = rangeTop + range.height()) + eleHeight - origEleHeight
          t = rangeBottom - origEleHeight
          if ele.css('position') is 'absolute'
            ele.stop(yes).animate
              'top': t
              'fast'
          else
            ele.css
              'top': t

        else
          eleTop = ele.offset().top
          t = if top > lastTop and top + viewHeight > eleTop + eleHeight
            top + viewHeight - eleHeight
          else if top < lastTop and top < eleTop
            top
          else
            undefined
          if t
            if ele.css('position') is 'absolute'
              ele.stop(yes).animate
                'top': t
                'fast'
            else
              ele.css
                'top': t

        ele.css
          'position': 'absolute'
          'display': 'inline'

      lastTop = top

    $(window).on 'scroll resize', ->
      clearTimeout n
      n = setTimeout scrolling, 50

    $ ->
      setTimeout scrolling, i * 500 for i in [1..8]

  scrollFollow '.sides',
    range: '.content'