SPIGOT-2211: Fix offhand slot client refresh

src/main/java/org/bukkit/craftbukkit/inventory/CraftInventoryPlayer.java

@@ -95,10 +95,13 @@ public class CraftInventoryPlayer extends CraftInventory implements org.bukkit.i
         // to reverse the order of the index from 8. That means we need 0 to correspond to 8, 1 to correspond to 7,
         // 2 to correspond to 6, and 3 to correspond to 5. We do this simply by taking the result of (index - 36) and
         // subtracting that value from 8.
-        if (index < PlayerInventory.getHotbarSize())
+        if (index < PlayerInventory.getHotbarSize()) {
-            index = index + 36;
+            index += 36;
-        else if (index > 35)
+        } else if (index > 39) {
+            index += 5; // Off hand
+        } else if (index > 35) {
             index = 8 - (index - 36);
+        }
         player.playerConnection.sendPacket(new PacketPlayOutSetSlot(player.defaultContainer.windowId, index, CraftItemStack.asNMSCopy(item)));
     }