I expect you're remembering User:Lupin/popups.js[1], which dates back to August 2005.

It soon became known as Navigation Popups[2], and is still available today.

The main documentation for Page Previews mentions Navigation Popups.[3]

[1]: https://en.wikipedia.org/wiki/User:Lupin/popups.js

[2]: https://en.wikipedia.org/wiki/Wikipedia:Tools/Navigation_pop...

[3]: https://www.mediawiki.org/wiki/Page_Previews