The Falcon Heavy test flight included a demonstration of capability to coast for several hours and then re-light the second stage for a final burn. During the coast, the cryo propellants are absorbing heat and slowly boiling off. The pressure in the tanks would be managed by venting. My question is: venting what? Only gas (ideally), or a potential mixture of gas and liquid? If gas only, how would that be accomplished in zero gravity? In other words, how would the liquid phase be conserved and only "boiled" gas released?
Different stages use different means; Centaur uses(used) a thermodynamic vent mixer system for its LH2 tank.
This device served to keep the bulk propellant well mixed, and ensured that only gas was vented.