It's not an Iframe. It's not even anything close to a frame. It's a Div element with a max-height of XX pixels (or em for lines) height and when there is overflow (text going over the max-height) it adds the scroll bars automatically.
You can test this out yourself:
<div style="width: 75px; height: 20px; max-height: 10em; overflow: auto; padding: 2px;">
<p style="width: 71px;">This is some content<br />
that flows past the maximum height<br />
of our content div.<br /><br />
When content expands below the<br />
div, a scrollbar is automatically added<br />
and it looks like a frame, but is really<br />
some good CSS 2 styling.<br /><br />
lorem ipsum dolor sit amet<br />
abcdefghijklmnopqrstuvwxyz<br />
now I know my abc's, next time<br />
won't you sing with me ;)<br />
End of lesson.</p>
</div>