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class Follower
(el) ->
@follower = el
@last-scroll-top = window.scroll-y
@initial-follower-top = @follower.get-bounding-client-rect!.top
@page-top = 48
follow: ->
window-height = window.inner-height
follower-height = @follower.offset-height
scroll-top = window.scroll-y
scroll-bottom = scroll-top + window-height
follower-top = @follower.get-bounding-client-rect!.top + scroll-top
follower-bottom = follower-top + follower-height
height-delta = Math.abs window-height - follower-height
scroll-delta = @last-scroll-top - scroll-top
is-scrolling-down = (scroll-top > @last-scroll-top)
is-window-larger = (window-height > follower-height)
console.log @initial-follower-top
if (is-window-larger && scroll-top > @initial-follower-top) || (!is-window-larger && scroll-top > @initial-follower-top + height-delta)
@follower.class-list.add \fixed
else if !is-scrolling-down && scroll-top + @page-top <= @initial-follower-top
@follower.class-list.remove \fixed
@follower.style.top = 0
return
drag-bottom-down = (follower-bottom <= scroll-bottom && is-scrolling-down)
drag-top-up = (follower-top >= scroll-top + @page-top && !is-scrolling-down)
if drag-bottom-down
console.log \down
@follower.style.top = if is-window-larger then 0 else -height-delta + \px
else if drag-top-up
console.log \up
@follower.style.top = @page-top + \px
else if @follower.class-list.contains \fixed
console.log \-
current-top = parse-int @follower.style.top, 10
min-top = -height-delta
scrolled-top = current-top + scroll-delta
is-page-at-bottom = (scroll-top + window-height >= document.body.offset-height)
new-top = if is-page-at-bottom then min-top else scrolled-top
@follower.style.top = new-top + \px
@last-scroll-top = scroll-top
module.exports = Follower
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