fix degenerate case

engine/physics/src/minimum_difference.rs

@@ -514,6 +514,10 @@ fn minimum_difference<const ENABLE_FAST_FAIL:bool,T>(
 	// local new_point_q = queryQ(-initialAxis)
 	// local direction, a0, a1, b0, b1, c0, c1, d0, d1
 	let mut direction=mesh.hint_point()+rel_pos;
+	// degenerate case
+	if direction==vec3::zero(){
+		direction=choose_any_direction();
+	}
 	let new_point=mesh.farthest_vert(direction);
 	let mut simplex=Simplex::from_iter([new_point]);