Light trips circuit breaker when connected to box
In this picture the black wire looks damaged right as it comes out of the fixture.  In other pictures you can clearly see that one of the mounting screw holes has electrical burn marks on it.

Because of the scorching, we know that one way or another, the hot side of the circuit found a path to ground through the fixture.  This is either because the wire going into the fixture is damaged, or something else is wrong.  If it was internal to the fixture, the ground wire should be causing the breaker to trip immediately, so I'm still thinking it's a damaged wire.

Warning - if the fixture trips the breaker when it's installed in the box, that means the entire fixture could be electrified when it's turned on.  Don't even touch the fixture when it's turned on.  I would make sure the breaker, not just the switch, is turned off before touching anything at all.