OpenSim​/Hack​/SIM_boundary

if (actor != null) { 
   actor.UpdatePositionAndVelocity();

   // by Fumi.Iseki  for falling at SIM boundary
   d.Vector3 vec = d.BodyGetPosition(actor.Body);

   if (vec.Y<=0.0f || vec.Y>=255.95f || vec.X<=0.0f || vec.X>=255.95f) {
       if      (vec.Y<=0.0f)   vec.Y = 0.1f; 
       else if (vec.Y>255.95f) vec.Y = 255.95f;
       if      (vec.X<=0.0f)   vec.X = 0.1f; 
       else if (vec.X>255.95f) vec.X = 255.95f;

       float height = GetTerrainHeightAtXY(Vec.X, Vec.Y);
       if (actor.Flying) {
           if (vec.Z  <  height + 2) {
               vec.Z = height + actor.Size.Z/2;
               actor.Velocity.Z = 0;
               actor.IsColliding = true; 
           }
       }
       else {
           if (vec.Z < height + actor.Size.Z/2) {
               vec.Z = height + actor.Size.Z/2;
               actor.Velocity.Z = 0;
               actor.IsColliding = true; 
               actor.CollidingGround = true; 
           }
       }

       actor.Position = new PhysicsVector(vec.X, vec.Y, vec.Z); 
       d.BodySetPosition(actor.Body, vec.X, vec.Y, vec.Z); 
   }       
}