Entities/Player: Clean up client control handling behavior around possession. Mind Control should no longer cause various weirdness.

Port From (https://github.com/TrinityCore/TrinityCore/commit/bba4696de715b79b31e154d1a8e1597507812ece)

Source/Game/Entities/Player/Player.cs

@@ -7214,24 +7214,38 @@ namespace Game.Entities
 
         public void SetClientControl(Unit target, bool allowMove)
         {
-            // still affected by some aura that shouldn't allow control, only allow on last such aura to be removed
-            if (allowMove && target.HasUnitState(UnitState.CantClientControl))
+            // a player can never client control nothing
+            Cypher.Assert(target);
+
+            // don't allow possession to be overridden
+            if (target.HasUnitState(UnitState.Charmed) && (GetGUID() != target.GetCharmerGUID()))
             {
                 // this should never happen, otherwise m_unitBeingMoved might be left dangling!
-                Cypher.Assert(GetUnitBeingMoved() == target);
+                Log.outError(LogFilter.Player, $"Player '{GetName()}' attempt to client control '{target.GetName()}', which is charmed by GUID {target.GetCharmerGUID()}");
                 return;
             }
 
+            // still affected by some aura that shouldn't allow control, only allow on last such aura to be removed
+            if (target.HasUnitState(UnitState.Controlled))
+                allowMove = false;
+
             ControlUpdate packet = new();
             packet.Guid = target.GetGUID();
             packet.On = allowMove;
             SendPacket(packet);
 
-            if (this != target)
-                SetViewpoint(target, allowMove);
+            WorldObject viewpoint = GetViewpoint();
+            if (viewpoint == null)
+                viewpoint = this;
+            if (target != viewpoint)
+            {
+                if (viewpoint != this)
+                    SetViewpoint(viewpoint, false);
+                if (target != this)
+                    SetViewpoint(target, true);
+            }
 
-            if (allowMove)
-                SetMovedUnit(target);
+            SetMovedUnit(target);
         }
 
         public Item GetWeaponForAttack(WeaponAttackType attackType, bool useable = false)

Source/Game/Entities/Unit/Unit.cs

@@ -172,7 +172,7 @@ namespace Game.Entities
             UpdateSplineMovement(diff);
             GetMotionMaster().Update(diff);
 
-            if (GetAI() == null && (!IsPlayer() || IsCharmed()))
+            if (GetAI() == null && (!IsPlayer() || (IsCharmed() && GetCharmerGUID().IsCreature())))
                 UpdateCharmAI();
 
             RefreshAI();