I was reading a review of another movie on the IMDB - I think the movie was from about 1903 or 1904, and it was claiming that that was the oldest slapstick movie. I suggested at the time there were three version of THE MILLER AND THE SWEEP made in 1897, and now I noticed I've seen this movie before and been amused by it. This has a couple of lovers kissing on the balcony, and then the guy is caught in a sack and beaten. That certainly sounds like slapstick to me.
In truth, trying to find a first in this field is rendered difficult by the question of what a motion picture is. Flipbooks have been around for hundreds of years. I'm just pleased to notice that Georges Hatot was doing something beyond tableaux vivante movies based on those massive David paintings by 1897.