Style overflow only if needed.

With ```overflow: scroll``` we get this:

![](https://i.cloudup.com/OhEpOe8ytn-2000x2000.png)

with ```overflow-y: scroll``` we get this:

![](https://i.cloudup.com/M19XoJEdQo-3000x3000.png)

with ```overflow-y: auto``` we get this:

![](https://i.cloudup.com/5UFl01IBOo-3000x3000.png)

I think the last is best hence the PR