- add +mnotvisible flag as per feature suggestion: https://forum.zdoom.org/viewtopic.php?t=77277

src/playsim/actor.h

@@ -413,6 +413,7 @@ enum ActorFlag8
 	MF8_STOPRAILS		= 0x00000200,	// [MC] Prevent rails from going further if an actor has this flag.
 	MF8_ABSVIEWANGLES	= 0x00000400,	// [MC] By default view angle/pitch/roll is an offset. This will make it absolute instead.
 	MF8_FALLDAMAGE		= 0x00000800,	// Monster will take fall damage regardless of map settings.
+	MF8_MNOTVISIBLE		= 0x00001000,	// Actor not visible to monsters
 	MF8_ALLOWTHRUBITS	= 0x00008000,	// [MC] Enable ThruBits property
 	MF8_FULLVOLSEE		= 0x00010000,	// Play see sound at full volume
 	MF8_E1M8BOSS		= 0x00020000,	// MBF21 boss death.

src/playsim/p_enemy.cpp

@@ -2344,7 +2344,7 @@ void A_DoChase (AActor *actor, bool fastchase, FState *meleestate, FState *missi
 
 	// [RH] Don't chase invisible targets
 	if (actor->target != NULL &&
-		actor->target->renderflags & RF_INVISIBLE &&
+		((actor->target->renderflags & RF_INVISIBLE) || (actor->target->flags8 & MF8_MNOTVISIBLE)) &&
 		actor->target != actor->goal)
 	{
 		actor->target = nullptr;

src/playsim/p_mobj.cpp

@@ -1681,6 +1681,7 @@ bool AActor::CanSeek(AActor *target) const
 	if ((flags2 & MF2_DONTSEEKINVISIBLE) && 
 		((target->flags & MF_SHADOW) || 
 		 (target->renderflags & RF_INVISIBLE) || 
+		 (target->flags8 & MF8_MNOTVISIBLE) ||
 		 !target->RenderStyle.IsVisible(target->Alpha)
 		)
 	   ) return false;

src/playsim/p_sight.cpp

@@ -866,7 +866,10 @@ sightcounts[0]++;
 //
 	// [RH] Andy Baker's stealth monsters:
 	// Cannot see an invisible object
-	if ((flags & SF_IGNOREVISIBILITY) == 0 && ((t2->renderflags & RF_INVISIBLE) || !t2->RenderStyle.IsVisible(t2->Alpha)))
+	if ((flags & SF_IGNOREVISIBILITY) == 0 &&
+		((t2->renderflags & RF_INVISIBLE) ||
+		(t2->flags8 & MF8_MNOTVISIBLE) ||
+		!t2->RenderStyle.IsVisible(t2->Alpha)))
 	{ // small chance of an attack being made anyway
 		if ((t1->Level->BotInfo.m_Thinking ? pr_botchecksight() : pr_checksight()) > 50)
 		{

src/scripting/thingdef_data.cpp

@@ -326,6 +326,7 @@ static FFlagDef ActorFlagDefs[]=
 	DEFINE_FLAG(MF8, RETARGETAFTERSLAM, AActor, flags8),
 	DEFINE_FLAG(MF8, STOPRAILS, AActor, flags8),
 	DEFINE_FLAG(MF8, FALLDAMAGE, AActor, flags8),
+	DEFINE_FLAG(MF8, MNOTVISIBLE, AActor, flags8),
 	DEFINE_FLAG(MF8, ABSVIEWANGLES, AActor, flags8),
 	DEFINE_FLAG(MF8, ALLOWTHRUBITS, AActor, flags8),
 	DEFINE_FLAG(MF8, FULLVOLSEE, AActor, flags8),