We need a word for that.
The argument that never took place, because someone bit their tongue and let the moment pass.
The confrontation that might have happened, but didn't, because someone decided it wasn't worth it, and turned out to be right.
The horn that wasn't honked.
A snarky comment left unposted.
The middle finger that kept its seat.
A fist left unthrown.
The war never started.
Vengeance left unsought.
An empty space in time, left unfilled by an offense no one remembers.
Thank God for these...whatever you call them.