Katy Perry has been a busy lady. In between platonic wedding-watching with Robert Pattinson, she managed to throw an A-list Memorial Day party -- and one of her invited guests was off-and-on boyfriend John Mayer.

Cue the "oh no, not again" eye rolling.

Us Weekly confirmed that Mayer was in attendance at Perry's Hollywood Hills bash, two months after their most recent split, with a source telling the magazine they "seem to be back together -- or back together-ish."

"They were very flirty and having a really good time together. They have good chemistry," said the insider, with another adding that the two "were definitely friendly."

Other attendees at the party included 'Girls' star Allison Williams and her boyfriend Ricky Van Veen -- a couple who previously doubled with Mayer and Perry -- as well as Mindy Kaling and BJ Novak.

Perry and Mayer called it off in March for the second time, but still can't seem to make it stick. Looks like she'll have no shortage of material to dish to Russell Brand about any time soon.