PaperMC/paper-server/patches/sources/net/minecraft/world/item/HangingEntityItem.java.patch

--- a/net/minecraft/world/item/HangingEntityItem.java
+++ b/net/minecraft/world/item/HangingEntityItem.java
@@ -66,6 +_,19 @@
 
             if (hangingEntity.survives()) {
                 if (!level.isClientSide) {
+                    // CraftBukkit start - fire HangingPlaceEvent
+                    org.bukkit.entity.Player who = (context.getPlayer() == null) ? null : (org.bukkit.entity.Player) context.getPlayer().getBukkitEntity();
+                    org.bukkit.block.Block blockClicked = level.getWorld().getBlockAt(blockPos.getX(), blockPos.getY(), blockPos.getZ());
+                    org.bukkit.block.BlockFace blockFace = org.bukkit.craftbukkit.block.CraftBlock.notchToBlockFace(clickedFace);
+                    org.bukkit.inventory.EquipmentSlot hand = org.bukkit.craftbukkit.CraftEquipmentSlot.getHand(context.getHand());
+
+                    org.bukkit.event.hanging.HangingPlaceEvent event = new org.bukkit.event.hanging.HangingPlaceEvent((org.bukkit.entity.Hanging) hangingEntity.getBukkitEntity(), who, blockClicked, blockFace, hand, org.bukkit.craftbukkit.inventory.CraftItemStack.asBukkitCopy(itemInHand));
+                    level.getCraftServer().getPluginManager().callEvent(event);
+
+                    if (event.isCancelled()) {
+                        return InteractionResult.FAIL;
+                    }
+                    // CraftBukkit end
                     hangingEntity.playPlacementSound();
                     level.gameEvent(player, GameEvent.ENTITY_PLACE, hangingEntity.position());
                     level.addFreshEntity(hangingEntity);