The following stealth changes are implemented.

- Combat no longer removes stealth, only damage does
 - Creatures will pursue a stealthed unit they cannot see if they're already in combat with them
 - When a player is ~~3 yards~~ 8% + 1.5 yards away from the usual stealth detection distance, the creature will perform the "alerted" effect.
 - When sitting/sleeping creatures are distracted or alerted, they will stand up
 - Idle movement creatures will return to their original (spawn) orientation after distract/alert
 - When entering combat with a distracted/alerted creature, distract state is removed
 - NPC no longer have a limit to stealth visibility (granted by stealth modifier spells/buffs)
 - If alert visibility is greater than aggro range, no alert sent

src/server/game/Entities/Unit/Unit.cpp

@@ -288,6 +288,28 @@ Unit::~Unit()
     ASSERT(m_dynObj.empty());
 }
 
+// Check if unit in combat with specific unit
+bool Unit::IsInCombatWith(Unit const* who) const
+{
+    // Check target exists
+    if (!who)
+        return false;
+
+    // Search in threat list
+    ObjectGuid guid = who->GetGUID();
+    for (ThreatContainer::StorageType::const_iterator i = m_ThreatManager.getThreatList().begin(); i != m_ThreatManager.getThreatList().end(); ++i)
+    {
+        HostileReference* ref = (*i);
+
+        // Return true if the unit matches
+        if (ref && ref->getUnitGuid() == guid)
+            return true;
+    }
+
+    // Nothing found, false.
+    return false;
+}
+
 void Unit::Update(uint32 p_time)
 {
     // WARNING! Order of execution here is important, do not change.
@@ -11885,8 +11907,8 @@ bool Unit::_IsValidAttackTarget(Unit const* target, SpellInfo const* bySpell, Wo
         if (IsOnVehicle(target) || m_vehicle->GetBase()->IsOnVehicle(target))
             return false;
 
-    // can't attack invisible (ignore stealth for aoe spells) also if the area being looked at is from a spell use the dynamic object created instead of the casting unit.
-    if ((!bySpell || !bySpell->HasAttribute(SPELL_ATTR6_CAN_TARGET_INVISIBLE)) && (obj ? !obj->CanSeeOrDetect(target, bySpell && bySpell->IsAffectingArea()) : !CanSeeOrDetect(target, bySpell && bySpell->IsAffectingArea())))
+    // can't attack invisible (ignore stealth for aoe spells) also if the area being looked at is from a spell use the dynamic object created instead of the casting unit. Ignore stealth if target is player and unit in combat with same player
+    if ((!bySpell || !bySpell->HasAttribute(SPELL_ATTR6_CAN_TARGET_INVISIBLE)) && (obj ? !obj->CanSeeOrDetect(target, bySpell && bySpell->IsAffectingArea()) : !CanSeeOrDetect(target, (bySpell && bySpell->IsAffectingArea()) || (target->GetTypeId() == TYPEID_PLAYER && target->HasStealthAura() && target->IsInCombat() && IsInCombatWith(target)))))
         return false;
 
     // can't attack dead