Core/AI: Finally move the "is creature engaged" flag to be a property of the creature AI, where it honestly always belonged.

Port From (https://github.com/TrinityCore/TrinityCore/commit/c13d83796f7b2111c5dcf8546bdd84eccd232ae3)

Source/Game/AI/CoreAI/CreatureAI.cs

@@ -29,6 +29,7 @@ namespace Game.AI
 {
     public class CreatureAI : UnitAI
     {
+        bool _isEngaged;
         bool _moveInLOSLocked;
         List<AreaBoundary> _boundary = new();
         bool _negateBoundary;
@@ -201,6 +202,12 @@ namespace Game.AI
             }
         }
 
+        public override void JustEnteredCombat(Unit who)
+        {
+            if (!IsEngaged() && !me.CanHaveThreatList())
+                EngagementStart(who);
+        }
+
         // Called for reaction at stopping attack at no attackers or targets
         public virtual void EnterEvadeMode(EvadeReason why = EvadeReason.Other)
         {
@@ -231,7 +238,7 @@ namespace Game.AI
 
         public bool UpdateVictim()
         {
-            if (!me.IsEngaged())
+            if (!IsEngaged())
                 return false;
 
             if (!me.HasReactState(ReactStates.Passive))
@@ -254,23 +261,53 @@ namespace Game.AI
             return true;
         }
 
+        public void EngagementStart(Unit who)
+        {
+            if (_isEngaged)
+            {
+                //Log.outError(LogFilter.ScriptsAi, $"CreatureAI::EngagementStart called even though creature is already engaged. Creature debug info:\n{me.GetDebugInfo()}");
+                return;
+            }
+            _isEngaged = true;
+
+            me.AtEngage(who);
+        }
+
+        public void EngagementOver()
+        {
+            if (!_isEngaged)
+            {
+                //Log.outError(LogFilter.ScriptsAi, $"CreatureAI::EngagementOver called even though creature is not currently engaged. Creature debug info:\n{me.GetDebugInfo()}");
+                return;
+            }
+            _isEngaged = false;
+
+            me.AtDisengage();
+        }
+
         public bool _EnterEvadeMode(EvadeReason why = EvadeReason.Other)
         {
-            if (!me.IsAlive())
+            if (me.IsInEvadeMode())
                 return false;
 
+            if (!me.IsAlive())
+            {
+                EngagementOver();
+                return false;
+            }
+
             me.RemoveAurasOnEvade();
 
             // sometimes bosses stuck in combat?
             me.CombatStop(true);
-            me.GetThreatManager().NotifyDisengaged();
             me.SetLootRecipient(null);
             me.ResetPlayerDamageReq();
             me.SetLastDamagedTime(0);
             me.SetCannotReachTarget(false);
             me.DoNotReacquireTarget();
+            EngagementOver();
 
-            return !me.IsInEvadeMode();
+            return true;
         }
 
         public CypherStrings VisualizeBoundary(int duration, Unit owner = null, bool fill = false)
@@ -404,11 +441,22 @@ namespace Game.AI
             me.DoImmediateBoundaryCheck();
         }
 
+        // Called for reaction whenever a new non-offline unit is added to the threat list
+        public virtual void JustStartedThreateningMe(Unit who)
+        {
+            if (!IsEngaged())
+                EngagementStart(who);
+        }
+
         // Called for reaction when initially engaged - this will always happen _after_ JustEnteredCombat
         public virtual void JustEngagedWith(Unit who) { }
 
         // Called when the creature is killed
-        public virtual void JustDied(Unit killer) { }
+        public virtual void JustDied(Unit killer) 
+        {
+            if (IsEngaged())
+                EngagementOver();
+        }
 
         // Called when the creature kills a unit
         public virtual void KilledUnit(Unit victim) { }
@@ -521,6 +569,8 @@ namespace Game.AI
         public virtual bool IsEscortNPC(bool onlyIfActive) { return false; }
 
         public List<AreaBoundary> GetBoundary() { return _boundary; }
+
+        public bool IsEngaged() { return _isEngaged; }
     }
 
     public class AISpellInfoType

Source/Game/AI/CoreAI/GuardAI.cs

@@ -48,7 +48,7 @@ namespace Game.AI
             {
                 me.GetMotionMaster().MoveIdle();
                 me.CombatStop(true);
-                me.GetThreatManager().NotifyDisengaged();
+                EngagementOver();
                 return;
             }
 
@@ -56,7 +56,7 @@ namespace Game.AI
 
             me.RemoveAllAuras();
             me.CombatStop(true);
-            me.GetThreatManager().NotifyDisengaged();
+            EngagementOver();
 
             me.GetMotionMaster().MoveTargetedHome();
         }

Source/Game/AI/CoreAI/PassiveAI.cs

@@ -77,6 +77,21 @@ namespace Game.AI
 
         public override void MoveInLineOfSight(Unit who) { }
 
+        public override void JustEnteredCombat(Unit who)
+        {
+            EngagementStart(who);
+        }
+
+        public override void JustExitedCombat()
+        {
+            EngagementOver();
+        }
+
+        public override void JustStartedThreateningMe(Unit who)
+        {
+
+        }
+
         public override void EnterEvadeMode(EvadeReason why) { }
     }
 
@@ -89,6 +104,8 @@ namespace Game.AI
 
         public override void MoveInLineOfSight(Unit unit) { }
         public override void AttackStart(Unit unit) { }
+        public override  void JustStartedThreateningMe(Unit unit)  { }
+        public override void JustEnteredCombat(Unit who) { }
         public override void UpdateAI(uint diff) { }
         public override void JustAppeared() { }
         public override void EnterEvadeMode(EvadeReason why) { }

Source/Game/AI/CoreAI/TotemAI.cs

@@ -21,7 +21,7 @@ using Game.Maps;
 
 namespace Game.AI
 {
-    public class TotemAI : CreatureAI
+    public class TotemAI : NullCreatureAI
     {
         ObjectGuid _victimGuid;
 
@@ -30,15 +30,6 @@ namespace Game.AI
             _victimGuid = ObjectGuid.Empty;
         }
 
-        public override void MoveInLineOfSight(Unit who) { }
-
-        public override void JustAppeared() { }
-
-        public override void EnterEvadeMode(EvadeReason why)
-        {
-            me.CombatStop(true);
-        }
-
         public override void UpdateAI(uint diff)
         {
             if (me.ToTotem().GetTotemType() != TotemType.Active)

Source/Game/AI/CoreAI/UnitAI.cs

@@ -536,7 +536,7 @@ namespace Game.AI
             _me = unit;
             _dist = dist;
             _playerOnly = playerOnly;
-            _exception = !withTank ? _me.GetThreatManager().GetCurrentVictim() : null;
+            _exception = !withTank ? unit.GetThreatManager().GetLastVictim() : null;
             _aura = aura;
         }
 

Source/Game/AI/ScriptedAI/ScriptedEscortAI.cs

@@ -135,9 +135,10 @@ namespace Game.AI
         {
             me.RemoveAllAuras();
             me.CombatStop(true);
-            me.GetThreatManager().NotifyDisengaged();
             me.SetLootRecipient(null);
 
+            EngagementOver();
+
             if (HasEscortState(EscortState.Escorting))
             {
                 AddEscortState(EscortState.Returning);

Source/Game/AI/ScriptedAI/ScriptedFollowerAI.cs

@@ -123,15 +123,19 @@ namespace Game.AI
         public override void EnterEvadeMode(EvadeReason why)
         {
             if (!me.IsAlive())
+            {
+                EngagementOver();
                 return;
+            }
 
             me.RemoveAllAuras();
             me.CombatStop(true);
-            me.GetThreatManager().NotifyDisengaged();
             me.SetLootRecipient(null);
             me.SetCannotReachTarget(false);
             me.DoNotReacquireTarget();
 
+            EngagementOver();
+
             if (HasFollowState(FollowState.Inprogress))
             {
                 Log.outDebug(LogFilter.Scripts, "FollowerAI left combat, returning to CombatStartPosition.");