Don't allow users to attempt to sit on objects in a child region without going to that region first.

If this is attempted, they get a "Try moving closer.  Can't sit on object because it is not in the same region as you." message instead, which is the same as current ll grid.
Sitting on ground is okay, since viewer navigates avatar to required region first before sitting.

OpenSim/Region/ClientStack/Linden/UDP/LLClientView.cs

@@ -6651,6 +6651,12 @@ namespace OpenSim.Region.ClientStack.LindenUDP
                 }
                 #endregion
 
+                if (SceneAgent.IsChildAgent)
+                {
+                    SendCantSitBecauseChildAgentResponse();
+                    return true;
+                }
+
                 AgentRequestSit handlerAgentRequestSit = OnAgentRequestSit;
 
                 if (handlerAgentRequestSit != null)
@@ -6675,6 +6681,12 @@ namespace OpenSim.Region.ClientStack.LindenUDP
                 }
                 #endregion
 
+                if (SceneAgent.IsChildAgent)
+                {
+                    SendCantSitBecauseChildAgentResponse();
+                    return true;
+                }
+
                 AgentSit handlerAgentSit = OnAgentSit;
                 if (handlerAgentSit != null)
                 {
@@ -6684,6 +6696,14 @@ namespace OpenSim.Region.ClientStack.LindenUDP
             return true;
         }
 
+        /// <summary>
+        /// Used when a child agent gets a sit response which should not be fulfilled.
+        /// </summary>
+        private void SendCantSitBecauseChildAgentResponse()
+        {
+            SendAlertMessage("Try moving closer.  Can't sit on object because it is not in the same region as you.");
+        }
+
         private bool HandleSoundTrigger(IClientAPI sender, Packet Pack)
         {
             SoundTriggerPacket soundTriggerPacket = (SoundTriggerPacket)Pack;

OpenSim/Region/Framework/Scenes/ScenePresence.cs

@@ -2297,6 +2297,9 @@ namespace OpenSim.Region.Framework.Scenes
 
         public void HandleAgentRequestSit(IClientAPI remoteClient, UUID agentID, UUID targetID, Vector3 offset)
         {
+            if (IsChildAgent)
+                return;
+
             if (ParentID != 0)
             {
                 if (ParentPart.UUID == targetID)
@@ -2523,6 +2526,9 @@ namespace OpenSim.Region.Framework.Scenes
 
         public void HandleAgentSit(IClientAPI remoteClient, UUID agentID)
         {
+            if (IsChildAgent)
+                return;
+
             SceneObjectPart part = m_scene.GetSceneObjectPart(m_requestedSitTargetID);
 
             if (part != null)
@@ -2583,6 +2589,9 @@ namespace OpenSim.Region.Framework.Scenes
 
         public void HandleAgentSitOnGround()
         {
+            if (IsChildAgent)
+                return;
+
 //            m_updateCount = 0;  // Kill animation update burst so that the SIT_G.. will stick..
             m_AngularVelocity = Vector3.Zero;
             Animator.TrySetMovementAnimation("SIT_GROUND_CONSTRAINED");