updated 07/27/2012 AT 5:00 PM ET
•originally published 07/27/2012 AT 5:30 PM ET
According to an onlooker, the pair “showed up as a couple and left as a couple,” on Thursday night at the Jobs movie wrap party at new L.A. restaurant Duplex.
At the 300-person fete, which included guests Diddy and Dermot Mulroney, the lovebirds, “appeared to be lighthearted and in good spirits, and hardly left each other’s side,” the source tells PEOPLE. “They stayed within five feet of each other practically the entire evening. It looked like young love.”
Linked since April, the former That ’70s Show costars, “were totally comfortable,” the insider says. “They were very friendly and spent a lot of the night engaging others in conversation while maintaining close and casual affection.”
During the party, that had guests feasting on hors d’oeuvres by Executive Chef Lauren Cartmel, Kunis sipped on Grey Goose and soda water, while her man had Grey Goose and tonic water, along with Pinot Noir. The duo stayed for a couple of hours.
– Jennifer Garcia