Unity Sweep Casts & Resting Contact

For custom (non-Rigidbody) flight/movement physics that sweeps a box each frame (Physics.BoxCastAll from current position along the frame's velocity) and stops/bounces on hit:

Gotcha: zero-distance "initial overlap" hits freeze a resting object

PhysX reports colliders the swept shape already overlaps at the sweep start as hits with distance = 0, point = Vector3.zero, and normal = -castDirection. Two consequences:

1. Garbage contact data. Reflecting velocity off that normal, or building an impact at that point, uses meaningless values (the "hit point" is the world origin).
2. Takeoff freeze. An object that settles into resting contact with the ground sees the floor as a zero-distance hit on every sweep — including one pointing straight up. If the collision response zeroes/reflects velocity on any hit, the object can never leave the ground: thrust integrates into velocity, the sweep "hits", velocity is cancelled, repeat. Symptom: motors/input clearly responding, object pinned in place.

Fix: skip hits with distance <= 0 in movement sweeps (genuine approaches always report a positive distance), and/or keep grounded objects parked on a small virtual floor above the surface so they never enter resting contact. The FPV mod does both — see BoxCastWorld and the pre-takeoff pad floor in FPV/src/FPVDroneManager.cs.

History: the FPV drone originally rested on a virtual pad floor (_launchStartWorldPos.y) and never touched terrain. A commit removed the clamp (it blocked diving below launch height off a cliff) and routed pre-liftoff ground contact through the normal cold-bounce/rest path — which hit this freeze and made the drone unable to take off at all. The repair kept the floor but releases it on horizontal clearance from the pad as well as vertical, and added the zero-distance skip.