It’s more maintainable to keep the code changes at the call site, that way if the behaviour of those variables changes it will be caught in the update process.

Thank you for fixing this, although the way you did it was not necessary. The 'p' method should be named 'ejectPassenger' or 'removePassenger' and the Entity parameter is the passenger being ejected/removed. In the context of the 'p' method 'this' is the vehicle ejecting it's passenger. My suggestion was not a "blind" assignment at all.

Technically speaking, you could fix this by just setting the vehicle back when either of the events are cancelled. But since thats a requirement I figured why not just set it before firing the events so we have visibility of it using Entity#getVehicle() in the events and then removing it if the events were never cancelled...

Vehicle event is broken too.

TestPlugin2.jar

public class TestPlugin extends JavaPlugin implements Listener {
    @Override
    public void onEnable() {
        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler
    public void on(VehicleExitEvent event) {
        event.setCancelled(true);
    }
}

Steps to reproduce:

1) lay down some rails

2) set minecart on rails

3) right click minecart to enter minecart

4) press shift to exit minecart

5) experience same thing (stuck in limbo, including all loss of control over minecart)

What about the vehicle event, its been cancellable for ages?

Try this one @md_5

1) Summon pig

2) Get carrot on a stick

3) Mount pig

4) Press shift to dismount pig

5) Make pig move using carrot on a stick

6) remove carrot on a stick from your hand

7) Watch as the server finally updates you back to your original position because you are stuck in limbo

I can confirm that there is something fishy going on here: On a tamed horse I can't open the inventory anymore after trying to dismount. I can still move it client side though but when I relog I join back to where I first mounted it.

Tested with villagers when I implemented the event and could not reproduce.

TestPlugin.jar

public class TestPlugin extends JavaPlugin implements Listener {
    @Override
    public void onEnable() {
        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler
    public void on(PlayerInteractAtEntityEvent event) {
        event.getRightClicked().addPassenger(event.getPlayer());
    }

    @EventHandler
    public void on(EntityDismountEvent event) {
        event.setCancelled(true);
    }
}

^{Steps to reproduce:}

^{1) Right click any entity to mount them (like a cow, or whatever)}

^{2) Press shift to dismount}

^{3) Experience being stuck in limbo. Cant move, cant dismount. (even the entity you mounted wont move. sometimes it looks like it tries, but its stuck, too)}

I tested this and did not see any issues with cancelling the dismount event.
Can you please be more specific about how to reproduce the issue; and why does that not affect the existing vehicle event.
Also that code to just blindly set .ax before firing the event and then remove it after looks incredibly suspicious to me.

Reopened for further evaluation.

This actually isn't finished. There's one important thing that wasn't implemented, and that's the "vehicle" gets lost (field Entity#ax).

To fix, you would simply have to re-set the vehicle, then if the events (both vehicle and dismount events) were not cancelled, set the vehicle back to null.

Here is a screenshot to aid in the implementation:

Without this, cancelling the dismount event will half eject the player, leaving them in a state of "being stuck" where they are in limbo. stuck between mounted and unmounted.