Paper/paper-server/nms-patches/net/minecraft/world/entity/EntityAreaEffectCloud.patch

--- a/net/minecraft/world/entity/EntityAreaEffectCloud.java
+++ b/net/minecraft/world/entity/EntityAreaEffectCloud.java
@@ -31,6 +31,12 @@
 import net.minecraft.world.level.material.EnumPistonReaction;
 import org.slf4j.Logger;
 
+// CraftBukkit start
+import net.minecraft.resources.MinecraftKey;
+import org.bukkit.craftbukkit.entity.CraftLivingEntity;
+import org.bukkit.entity.LivingEntity;
+// CraftBukkit end
+
 public class EntityAreaEffectCloud extends Entity implements TraceableEntity {
 
     private static final Logger LOGGER = LogUtils.getLogger();
@@ -44,7 +50,7 @@
     private static final float DEFAULT_RADIUS = 3.0F;
     public static final float DEFAULT_WIDTH = 6.0F;
     public static final float HEIGHT = 0.5F;
-    private PotionRegistry potion;
+    public PotionRegistry potion; // CraftBukkit - private -> public
     public List<MobEffect> effects;
     private final Map<Entity, Integer> victims;
     private int duration;
@@ -112,7 +118,7 @@
 
     }
 
-    private void updateColor() {
+    public void updateColor() { // Craftbukkit - private -> public
         if (this.potion == Potions.EMPTY && this.effects.isEmpty()) {
             this.getEntityData().set(EntityAreaEffectCloud.DATA_COLOR, 0);
         } else {
@@ -263,6 +269,7 @@
                     if (!list1.isEmpty()) {
                         Iterator iterator1 = list1.iterator();
 
+                        List<LivingEntity> entities = new java.util.ArrayList<LivingEntity>(); // CraftBukkit
                         while (iterator1.hasNext()) {
                             EntityLiving entityliving = (EntityLiving) iterator1.next();
 
@@ -272,6 +279,17 @@
                                 double d8 = d6 * d6 + d7 * d7;
 
                                 if (d8 <= (double) (f * f)) {
+                                    // CraftBukkit start
+                                    entities.add((LivingEntity) entityliving.getBukkitEntity());
+                                }
+                            }
+                        }
+                        org.bukkit.event.entity.AreaEffectCloudApplyEvent event = org.bukkit.craftbukkit.event.CraftEventFactory.callAreaEffectCloudApplyEvent(this, entities);
+                        if (!event.isCancelled()) {
+                            for (LivingEntity entity : event.getAffectedEntities()) {
+                                if (entity instanceof CraftLivingEntity) {
+                                    EntityLiving entityliving = ((CraftLivingEntity) entity).getHandle();
+                                    // CraftBukkit end
                                     this.victims.put(entityliving, this.tickCount + this.reapplicationDelay);
                                     Iterator iterator2 = list.iterator();
 
@@ -281,7 +299,7 @@
                                         if (mobeffect1.getEffect().isInstantenous()) {
                                             mobeffect1.getEffect().applyInstantenousEffect(this, this.getOwner(), entityliving, mobeffect1.getAmplifier(), 0.5D);
                                         } else {
-                                            entityliving.addEffect(new MobEffect(mobeffect1), this);
+                                            entityliving.addEffect(new MobEffect(mobeffect1), this, org.bukkit.event.entity.EntityPotionEffectEvent.Cause.AREA_EFFECT_CLOUD); // CraftBukkit
                                         }
                                     }