These little flies were fighting, it seems, for dominance or territory. When two of these get too close, they spread their wings, face each other, and eventually have a quick physical skirmish. The loser gets chased off the leaf and the victor gets to stay...until the next one comes along. A game of king of the mountain (or plant in this case). And I sometimes saw a smaller fly fend off a bigger one. These flies are pretty aggressive.