Ooooo ... never saw them do that before! While I was typing, caught white motion outside the window. Another jack was creeping up behind Chocolate-face, close enough for tail-sniffing. Chocolate-Face whirled around and lunged at the intruder, who dashed off in a small circle and came back face-to-face. They did the challenge stare, the standing up tall on extended legs with backs arched like angry cats that I've seen before, and then they both half-reared up and boxed at each other with their front paws for the few moments before gravity noticed and brought them back down (that's the new-to-me bit). Chocolate-Face won ... the other ran away as soon as front paws touched the ground.
More exciting play-by-play if it happens when I'm looking!