PaperMC/paper-server/nms-patches/net/minecraft/world/item/ItemLeash.patch

--- a/net/minecraft/world/item/ItemLeash.java
+++ b/net/minecraft/world/item/ItemLeash.java
@@ -15,6 +15,11 @@
 import net.minecraft.world.level.gameevent.GameEvent;
 import net.minecraft.world.phys.AxisAlignedBB;
 
+// CraftBukkit start
+import org.bukkit.craftbukkit.CraftEquipmentSlot;
+import org.bukkit.event.hanging.HangingPlaceEvent;
+// CraftBukkit end
+
 public class ItemLeash extends Item {
 
     public ItemLeash(Item.Info item_info) {
@@ -31,7 +36,7 @@
             EntityHuman entityhuman = itemactioncontext.getPlayer();
 
             if (!world.isClientSide && entityhuman != null) {
-                bindPlayerMobs(entityhuman, world, blockposition);
+                bindPlayerMobs(entityhuman, world, blockposition, itemactioncontext.getHand()); // CraftBukkit - Pass hand
             }
 
             return EnumInteractionResult.sidedSuccess(world.isClientSide);
@@ -40,7 +45,7 @@
         }
     }
 
-    public static EnumInteractionResult bindPlayerMobs(EntityHuman entityhuman, World world, BlockPosition blockposition) {
+    public static EnumInteractionResult bindPlayerMobs(EntityHuman entityhuman, World world, BlockPosition blockposition, net.minecraft.world.EnumHand enumhand) { // CraftBukkit - Add EnumHand
         EntityLeash entityleash = null;
         boolean flag = false;
         double d0 = 7.0D;
@@ -56,9 +61,26 @@
             if (entityinsentient.getLeashHolder() == entityhuman) {
                 if (entityleash == null) {
                     entityleash = EntityLeash.getOrCreateKnot(world, blockposition);
+
+                    // CraftBukkit start - fire HangingPlaceEvent
+                    org.bukkit.inventory.EquipmentSlot hand = CraftEquipmentSlot.getHand(enumhand);
+                    HangingPlaceEvent event = new HangingPlaceEvent((org.bukkit.entity.Hanging) entityleash.getBukkitEntity(), entityhuman != null ? (org.bukkit.entity.Player) entityhuman.getBukkitEntity() : null, world.getWorld().getBlockAt(i, j, k), org.bukkit.block.BlockFace.SELF, hand);
+                    world.getCraftServer().getPluginManager().callEvent(event);
+
+                    if (event.isCancelled()) {
+                        entityleash.discard(null); // CraftBukkit - add Bukkit remove cause
+                        return EnumInteractionResult.PASS;
+                    }
+                    // CraftBukkit end
                     entityleash.playPlacementSound();
                 }
 
+                // CraftBukkit start
+                if (org.bukkit.craftbukkit.event.CraftEventFactory.callPlayerLeashEntityEvent(entityinsentient, entityleash, entityhuman, enumhand).isCancelled()) {
+                    continue;
+                }
+                // CraftBukkit end
+
                 entityinsentient.setLeashedTo(entityleash, true);
                 flag = true;
             }
@@ -70,4 +92,10 @@
 
         return flag ? EnumInteractionResult.SUCCESS : EnumInteractionResult.PASS;
     }
+
+    // CraftBukkit start
+    public static EnumInteractionResult bindPlayerMobs(EntityHuman entityhuman, World world, BlockPosition blockposition) {
+        return bindPlayerMobs(entityhuman, world, blockposition, net.minecraft.world.EnumHand.MAIN_HAND);
+    }
+    // CraftBukkit end
 }