- add +VISALWAYSFAIL flag

src/playsim/actor.h

@@ -414,6 +414,7 @@ enum ActorFlag8
 	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_VISALWAYSFAIL	= 0x00002000,	// Sight checks to actor always fail
 	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

@@ -1252,7 +1252,7 @@ int P_IsVisible(AActor *lookee, AActor *other, INTBOOL allaround, FLookExParams
 	double mindist;
 	DAngle fov;
 
-	if (other == nullptr)
+	if (other == nullptr || (other->flags8 & MF8_VISALWAYSFAIL))
 	{
 		return false;
 	}

src/playsim/p_sight.cpp

@@ -849,6 +849,11 @@ int P_CheckSight (AActor *t1, AActor *t2, int flags)
 		return false;
 	}
 
+	if (t2->flags8 & MF8_VISALWAYSFAIL)
+	{
+		return false;
+	}
+
 	auto s1 = t1->Sector;
 	auto s2 = t2->Sector;
 	//

src/scripting/thingdef_data.cpp

@@ -327,6 +327,7 @@ static FFlagDef ActorFlagDefs[]=
 	DEFINE_FLAG(MF8, STOPRAILS, AActor, flags8),
 	DEFINE_FLAG(MF8, FALLDAMAGE, AActor, flags8),
 	DEFINE_FLAG(MF8, MNOTVISIBLE, AActor, flags8),
+	DEFINE_FLAG(MF8, VISALWAYSFAIL, AActor, flags8),
 	DEFINE_FLAG(MF8, ABSVIEWANGLES, AActor, flags8),
 	DEFINE_FLAG(MF8, ALLOWTHRUBITS, AActor, flags8),
 	DEFINE_FLAG(MF8, FULLVOLSEE, AActor, flags8),