Dimension travel

AkumaDimensionTravel moves a body — and whatever it is riding, and whoever is riding it — from one dimension to another, with a velocity that survives the trip.

AkumaDimensionTravel.preload(sky, target);   // while the ride up is still playing
AkumaDimensionTravel.send(player, sky, target, new Vec3(0, 1.2, 0));

AkumaDimensionTravel.launch(entity, new Vec3(dx, 1.6, dz));   // a push that sticks

The five traps it carries

None of them looks like a bug until somebody is in the air.

⚠️ changeDimension dismounts passengers. A player in a boat arrives without the boat, and the boat arrives without the player — or does not arrive at all. The vehicle goes first, the riders follow, and they are put back together on the next tick: a passenger mounted during the transfer tick is dropped again by the tail of the teleport.

⚠️ The client resets velocity on transfer. Whatever the server set is gone the moment the player loads in, so the motion packet is resent after arrival. Without it a launch upward ends in a standing start.

⚠️ A ridden vehicle's velocity is owned by the rider's client. Setting it server-side alone does nothing — the client's next movement packet overrules it. launch sends ClientboundSetEntityMotionPacket to the riders too, which is what makes a boat in a column actually rise. This is the case every hand-written copy misses, and it is also why AkumaMotion states plainly that it does not sync a body carrying a rider: that helper is about pulls, this one is about launches, and only the second has to fight the rider's client.

⚠️ Generating the destination chunk during the transfer freezes the server. preload exists to be called earlier, while whatever is carrying the player is still playing out. It blocks until the chunk is ready, which is exactly why it must not happen at the moment of transfer.

⚠️ The arrival chunk is held open across the transfer. Vanilla adds a POST_TELEPORT ticket for a player, in ServerPlayer.teleportTo, and for nothing else — so a boat or a mob sent to another dimension is handed to a chunk that is free to unload in the same tick. It is not lost, but it is in no loaded chunk: it ticks for nobody, answers no query, and reappears only when something else happens to load that chunk. That reads exactly like an entity that vanished, and it is what a game test saw before this was added.

Fall distance is cleared on arrival, or a launch turns into a death.

What send returns

The entity as it exists in the destination — changeDimension returns a new instance, not the one you passed. Pass a rider or its vehicle; either way the whole stack moves, and you get back the counterpart of whatever you handed in.

What it is not

It does not decide where to arrive. The caller gives a target that it has already checked, because "is there ground there?" is the consumer's question: a sky dimension wants a cloud under the player, a cave dimension wants air.