JESS: Seemed to me like you’re still pretty together. I half expected you to break into a barn and put on a show.
Jess is referring to the 1950 musical film Summer Stock, directed by Charles Walters, starring Judy Garland, and previously mentioned. In the film, Judy Garland’s character, Jane, owns a farm, and one day her actress sister Abigail (played by Gloria DeHaven) breaks into her barn to rehearse with her fiancé, theatre director Joe (played by Gene Kelly), along with their theatre troupe.
Jane reluctantly allows them to go ahead, and inevitably gets sucked into the action by taking part in their show, including their big song and dance numbers. By the end, Jane has dumped her boring farmboy fiance Orville (played by Eddie Bracken), and winds up with Joe, while Abigail has, slightly implausibly, been smitten with Orville. The film was a box office success, and is iconic in popular culture, often referenced in songs and music videos.
Note that Jess likens Rory and Dean to a couple who break up to find different partners – this is essentially what ends up happening.