Because of the extra padding that CM places on the scrollbar div, I'd have to do something like this:

```
                var sbWidth = $sb[0].offsetWidth - 1;  // CM adds an extra 1px space next to sb
                trackOffset = sbWidth;  // Up arrow pushes down track (assume arrow ht = sb width)
```

...which seems not necessarily less fragile than a magic number.  So I'm sort of inclined to just hardcode it in a const, if you're ok with it.