Paper/patches/server/0796-Sync-offhand-slot-in-menus.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Jake Potrebic <jake.m.potrebic@gmail.com>
Date: Fri, 14 Jan 2022 10:20:40 -0800
Subject: [PATCH] Sync offhand slot in menus

Menus don't add slots for the offhand, so on sendAllDataToRemote calls the
offhand slot isn't sent. This is not correct because you *can* put stuff into the offhand
by pressing the offhand swap item

diff --git a/src/main/java/net/minecraft/server/level/ServerPlayer.java b/src/main/java/net/minecraft/server/level/ServerPlayer.java
index d139e9e8b83ee4f075228ef0d42786c470012c2d..992032821110fc658fecc09530097a526f20d74d 100644
--- a/src/main/java/net/minecraft/server/level/ServerPlayer.java
+++ b/src/main/java/net/minecraft/server/level/ServerPlayer.java
@@ -316,6 +316,13 @@ public class ServerPlayer extends Player {
 
             }
 
+            // Paper start - Sync offhand slot in menus
+            @Override
+            public void sendOffHandSlotChange() {
+                ServerPlayer.this.connection.send(new ClientboundContainerSetSlotPacket(ServerPlayer.this.inventoryMenu.containerId, ServerPlayer.this.inventoryMenu.incrementStateId(), net.minecraft.world.inventory.InventoryMenu.SHIELD_SLOT, ServerPlayer.this.inventoryMenu.getSlot(net.minecraft.world.inventory.InventoryMenu.SHIELD_SLOT).getItem().copy()));
+            }
+            // Paper end - Sync offhand slot in menus
+
             @Override
             public void sendSlotChange(AbstractContainerMenu handler, int slot, ItemStack stack) {
                 ServerPlayer.this.connection.send(new ClientboundContainerSetSlotPacket(handler.containerId, handler.incrementStateId(), slot, stack));
diff --git a/src/main/java/net/minecraft/world/inventory/AbstractContainerMenu.java b/src/main/java/net/minecraft/world/inventory/AbstractContainerMenu.java
index ea84d4549815ddfd2829cb2fd3885ddd1114bfa8..85fb19177690ea7235c10f64789066599db08b05 100644
--- a/src/main/java/net/minecraft/world/inventory/AbstractContainerMenu.java
+++ b/src/main/java/net/minecraft/world/inventory/AbstractContainerMenu.java
@@ -200,6 +200,7 @@ public abstract class AbstractContainerMenu {
 
         if (this.synchronizer != null) {
             this.synchronizer.sendInitialData(this, this.remoteSlots, this.remoteCarried, this.remoteDataSlots.toIntArray());
+            this.synchronizer.sendOffHandSlotChange(); // Paper - Sync offhand slot in menus; update player's offhand since the offhand slot is not added to the slots for menus but can be changed by swapping from a menu slot
         }
 
     }
diff --git a/src/main/java/net/minecraft/world/inventory/ContainerSynchronizer.java b/src/main/java/net/minecraft/world/inventory/ContainerSynchronizer.java
index ff4fa86f9408e83e505f7e27692d3423f8570c48..a45ef5fcffc05e4e30801b73e82d29c6dbf5b8fd 100644
--- a/src/main/java/net/minecraft/world/inventory/ContainerSynchronizer.java
+++ b/src/main/java/net/minecraft/world/inventory/ContainerSynchronizer.java
@@ -6,6 +6,7 @@ import net.minecraft.world.item.ItemStack;
 public interface ContainerSynchronizer {
     void sendInitialData(AbstractContainerMenu handler, NonNullList<ItemStack> stacks, ItemStack cursorStack, int[] properties);
 
+    default void sendOffHandSlotChange() {} // Paper - Sync offhand slot in menus
     void sendSlotChange(AbstractContainerMenu handler, int slot, ItemStack stack);
 
     void sendCarriedChange(AbstractContainerMenu handler, ItemStack stack);