use overflow: auto

`overflow: scroll` forces scrollbars _always_, even when not needed.
`overflow: auto` uses scrollbars _only when needed_ (scrollable content
hidden away).

IMPORTANT NOTE:
OS X hides scrollbars when using trackpads. To see what is really there,
you have to disable this hiding. to do so:

    System Prefs > General > "show scrollbars: Always".

This shows what scrollbars really are there or not. When using
`overflow: scroll`, the scrollbar space will always be there, but OS X
hides it away so you can't see it. But everyone not on OS X + trackpad,
sees the horrible space.