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

paper-server/patches/sources/net/minecraft/world/inventory/AbstractContainerMenu.java.patch

@@ -61,7 +61,12 @@
  
      protected AbstractContainerMenu(@Nullable MenuType<?> type, int syncId) {
          this.carried = ItemStack.EMPTY;
-@@ -192,6 +232,15 @@
+@@ -188,10 +228,20 @@
+ 
+         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
+         }
  
      }
  
@@ -77,7 +82,7 @@
      public void removeSlotListener(ContainerListener listener) {
          this.containerListeners.remove(listener);
      }
-@@ -281,7 +330,7 @@
+@@ -281,7 +331,7 @@
              while (iterator.hasNext()) {
                  ContainerListener icrafting = (ContainerListener) iterator.next();
  
@@ -86,7 +91,7 @@
              }
          }
  
-@@ -417,7 +466,7 @@
+@@ -417,7 +467,7 @@
                  }
              } else if (this.quickcraftStatus == 2) {
                  if (!this.quickcraftSlots.isEmpty()) {
@@ -95,7 +100,7 @@
                          k = ((Slot) this.quickcraftSlots.iterator().next()).index;
                          this.resetQuickCraft();
                          this.doClick(k, this.quickcraftType, ClickType.PICKUP, player);
-@@ -433,6 +482,7 @@
+@@ -433,6 +483,7 @@
                      l = this.getCarried().getCount();
                      Iterator iterator = this.quickcraftSlots.iterator();
  
@@ -103,7 +108,7 @@
                      while (iterator.hasNext()) {
                          Slot slot1 = (Slot) iterator.next();
                          ItemStack itemstack2 = this.getCarried();
-@@ -443,12 +493,48 @@
+@@ -443,12 +494,48 @@
                              int l1 = Math.min(AbstractContainerMenu.getQuickCraftPlaceCount(this.quickcraftSlots, this.quickcraftType, itemstack1) + j1, k1);
  
                              l -= l1 - j1;
@@ -155,7 +160,7 @@
                  }
  
                  this.resetQuickCraft();
-@@ -466,8 +552,11 @@
+@@ -466,8 +553,11 @@
                  if (slotIndex == -999) {
                      if (!this.getCarried().isEmpty()) {
                          if (clickaction == ClickAction.PRIMARY) {
@@ -168,7 +173,7 @@
                          } else {
                              player.drop(this.getCarried().split(1), true);
                          }
-@@ -530,6 +619,15 @@
+@@ -530,6 +620,15 @@
                      }
  
                      slot.setChanged();
@@ -184,7 +189,7 @@
                  }
              } else {
                  int j2;
-@@ -662,8 +760,9 @@
+@@ -662,8 +761,9 @@
              ItemStack itemstack = this.getCarried();
  
              if (!itemstack.isEmpty()) {
@@ -195,7 +200,7 @@
              }
  
          }
-@@ -893,6 +992,11 @@
+@@ -893,6 +993,11 @@
      }
  
      public ItemStack getCarried() {

paper-server/patches/sources/net/minecraft/world/inventory/ContainerSynchronizer.java.patch

@@ -0,0 +1,10 @@
+--- a/net/minecraft/world/inventory/ContainerSynchronizer.java
++++ b/net/minecraft/world/inventory/ContainerSynchronizer.java
+@@ -6,6 +6,7 @@
+ 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);