Happened to have?

Hi! everyone Could you proofread my sentences and my understandings about them.
1-Sally and John are at draggers drawn.Last night Sally went to the same party where John happened to have come( about the coloured sentence,according to me it means John had arrived at the party before Sally arrived)
If above isn't right.Should I say (where John happened to come) ?According to me it means John came as soon as the Sally arrived or John arrived after her. Am I right?
And please also could you tell me the difference between "It just so happens that and It so happens that" (according to me these both mean "Coincidentally" Am I right here?
Thanks!