1 files changed, 56 insertions, 0 deletions
diff --git a/src/web/app/desktop/scripts/follow-scroll.ls b/src/web/app/desktop/scripts/follow-scroll.ls
new file mode 100644
index 0000000000..5072e9c583
--- /dev/null
+++ b/src/web/app/desktop/scripts/follow-scroll.ls
@@ -0,0 +1,56 @@
+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