[Mediawiki-l] Automatic horizontal scrollbar in <pre> on mediawiki.org?

[Daniel Barrett]

In <pre>...</pre> sections on mediawiki.org, a horizontal scrollbar is
automatically rendered when the lines are long.  Is this a built-in
MediaWiki feature, or an extension?  It doesn't happen on Wikipedia, and
I don't see anything likely in mediawiki.org's extension list in
Special:Version, nor in mediawiki:common.{css,js}.

Thanks,
DanB