ぼくがかんがえたさいきょうのスクロール追従メニュー

jade

.fixed
  p.pagetop
    a(href='#top')
      img(src='pagetop.gif', alt='トップへ戻る')

coffee

$ ->
  wantToShow = 85

  $(window).scroll(()->
    percentage = $(this).scrollTop() / ($(document).height() - $(this).height()) * 100

    if percentage > wantToShow
      $('.fixed').addClass('is-active')
    else
      $('.fixed').removeClass('is-active')
  )

scss

$mouseover:.15s;

.fixed {
    @include transition(opacity, $mouseover, linear);
    @include opacity(0);
    position: fixed;
    right: 18px;
    bottom: 200px;
    z-index: -1;

    &.is-active {
        @include opacity(1);
        z-index: 1;
    }
}