Fix smooth scroll to bottom never timing out

Two issues:
  - timing @stop! didn't actually stop @auto repeating.
  - Once the scroll hit bottom, it never snopped the timeout.
    I suppose I could auto: false and change the scroller function to
    explicitly repeat instead of having two "stop" calls...
Simplify smooth scroll to bottom function

I still don't really like early "return unless"'s, even though I could
avoid an indentation block. They just are too suprising if you're not
looking for them, especially when hidden with a postfix conditional.