Some tweaks to simplify firing MobSpawnEvent.FinalizeSpawn from Kilt

src/main/java/net/minecraftforge/event/ForgeEventFactory.java

@@ -82,9 +82,7 @@
 import org.jetbrains.annotations.Nullable;
 
 import java.io.File;
-import java.util.EnumSet;
-import java.util.List;
-import java.util.Optional;
+import java.util.*;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.BooleanSupplier;
 import java.util.function.Consumer;
@@ -184,6 +182,25 @@ public static boolean checkSpawnPositionSpawner(Mob mob, ServerLevelAccessor lev
         return event.getResult() == Result.ALLOW;
     }
 
+    // We insert this into onFinalizeSpawn with a mixin for best compatibility.
+    private static final ThreadLocal<Operation<SpawnGroupData>> kilt$fabricOriginal = ThreadLocal.withInitial(() -> null);
+
+    public static final ThreadLocal<Set<Mob>> kilt$hasFiredInitializeEvent = ThreadLocal.withInitial(HashSet::new);
+
+    // Ideally, I would have wanted to turn onFinalizeSpawn into a stub calling this.
+    // Unfortunately: https://github.com/The-Aether-Team/The-Aether/blob/1.20.1-develop/src/main/java/com/aetherteam/aether/mixin/mixins/common/ForgeEventFactoryMixin.java#L24-L29
+    public static SpawnGroupData kilt$onFinalizeSpawn(
+            Mob mob, ServerLevelAccessor level, DifficultyInstance difficulty, MobSpawnType spawnType,
+            @Nullable SpawnGroupData spawnData, @Nullable CompoundTag spawnTag, Operation<SpawnGroupData> original
+    ) {
+        try {
+            kilt$fabricOriginal.set(original);
+            return onFinalizeSpawn(mob, level, difficulty, spawnType, spawnData, spawnTag);
+        } finally {
+            kilt$fabricOriginal.set(null);
+        }
+    } // Removing would likely just cause unnecessary lag.
+
     /**
      * Vanilla calls to {@link Mob#finalizeSpawn} are replaced with calls to this method via coremod.<br>
      * Mods should call this method in place of calling {@link Mob#finalizeSpawn}. Super calls (from within overrides) should not be wrapped.
@@ -218,15 +235,20 @@ public static boolean checkSpawnPositionSpawner(Mob mob, ServerLevelAccessor lev
     @SuppressWarnings("deprecation") // Call to deprecated Mob#finalizeSpawn is expected.
     public static SpawnGroupData onFinalizeSpawn(Mob mob, ServerLevelAccessor level, DifficultyInstance difficulty, MobSpawnType spawnType, @Nullable SpawnGroupData spawnData, @Nullable CompoundTag spawnTag)
     {
-        var event = new MobSpawnEvent.FinalizeSpawn(mob, level, mob.getX(), mob.getY(), mob.getZ(), difficulty, spawnType, spawnData, spawnTag, null);
-        boolean cancel = MinecraftForge.EVENT_BUS.post(event);
+        try {
+            var event = new MobSpawnEvent.FinalizeSpawn(mob, level, mob.getX(), mob.getY(), mob.getZ(), difficulty, spawnType, spawnData, spawnTag, null);
+            boolean cancel = MinecraftForge.EVENT_BUS.post(event);
+            kilt$hasFiredInitializeEvent.get().add(mob);
 
-        if (!cancel)
-        {
-            return mob.finalizeSpawn(level, event.getDifficulty(), event.getSpawnType(), event.getSpawnData(), event.getSpawnTag());
-        }
+            if (!cancel)
+            {
+                return mob.finalizeSpawn(level, event.getDifficulty(), event.getSpawnType(), event.getSpawnData(), event.getSpawnTag());
+            }
 
-        return null;
+            return null;
+        } finally {
+            kilt$hasFiredInitializeEvent.get().remove(mob);
+        }
     }
 
     /**