Newton's first law of motion answers your question, the principle of inertia. If the car is at constant velocity everything inside the car will follow with the same velocity. If no other forces are applied to the person, once it jumps, it will still have the car's velocity due to inertia and will land at the exact same spot over the car. It doesn't matter what kind of situation it is, if you can assure there is no net forces acting, the result of the jump will be the same.
If the car is accelerating, however, it changes. If the car is gaining speed, then once the person jumps, it will lag behind the car because the car is still gaining speed while the person isn't. If the car is losing speed, once the person jumps it will get a lead over the car.