Ah, this sort of addresses my previous comment about bailing if there is no `parent`. But since `document.getElementById('scrollbar-parent')` is `null` (not `undefined`) if the element isn't present, this check will pass unexpectedly.

I would just change these `if`s to `if (scrollList)` and `if (parent)`. That way you cover your bases for `null` and `undefined` (and empty string, etc). But even with that change, you might want to guard against `parent` being null in scroll-list.js.