Whoa!!! We NEVER expected THIS! Kourtney Kardashian actually hung out with baby daddy Scott Disick and his much-younger girlfriend Sofia Richie!

The group were all invited, along with Kim Kardashian and pals Larsa Pippen and Brittny Gastineau, to the Sweet Dreams gallery exhibit at Craig's Friday night. Photos from inside the event suggest there wasn't much overlap between Kourt and her 20-year-old replacement but sources say the two were cordial.

The two were having a sexy fashion face-off -- Kourtney in her VERY low-cut crop top and Sofia in a skin-tight olive mini-dress exposing her long legs. We'd say it was a tie ...

Scott and Kourtney recently went with the rest of the family to Bali for a Kardashian clan vacation and Kourt's on-again-off-again relationship with much-younger Luka Sabbat seems to have cooled -- perhaps he was just a rebound after her breakup with long-time beau Younes Bendjima.

And amid all this dramz, another funny thing happened ... Ms. Gastineau, Kim's former BFF, was posing with her mom Lisa and a pal as they entered the event and her low-cut blazer with NOTHING underneath got REALLY revealing! Watch!!!