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;
	}
}