Cavanaugh gives the play-by-play of how the magical moment went down to Refinery29, via email. She says Bieber arrived first and began chatting with her and her friends."He mentioned he was waiting for his ex-girlfriend and was excited [and] nervous. We all threw out the 'man, we've all been there' kind of thing and tried to just talk normally — if that's even a thing," she wrote.Soon after, Gomez rolled in solo. The pair sat together and later danced to Lionel Richie's "Easy" before Bieber hit the keys."When he sang 'My Girl' to her, he stood with the mic and somehow invited everyone into this little moment between two people that clearly admire each other very much," Cavanaugh said.The Beliebers in the audience weren't the only ones who loved the performance. Cavanaugh said Gomez was smiling and laughing the entire time.Your guess is as good as ours as to whether the public display of cute means anything. What we do know is, the on-again, off-again duo sure do enjoy each another's company — both out on the town and in the recording studio. Bieber and Gomez released a surprise, stripped-down duet , "Strong," in October. Be it literally or figuratively, the two make beautiful music together.