Send the set item packet to the client when an item is changed or added in a player's inventory - Addresses SPIGOT-883

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

@@ -310,11 +310,15 @@ public class CraftInventory implements Inventory {
                     // Check if it fully fits
                     if (amount + partialAmount <= maxAmount) {
                         partialItem.setAmount(amount + partialAmount);
+                        // To make sure the packet is sent to the client
+                        setItem(firstPartial, partialItem);
                         break;
                     }
 
                     // It fits partially
                     partialItem.setAmount(maxAmount);
+                    // To make sure the packet is sent to the client
+                    setItem(firstPartial, partialItem);
                     item.setAmount(amount + partialAmount - maxAmount);
                 }
             }

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

@@ -1,6 +1,8 @@
 package org.bukkit.craftbukkit.inventory;
 
+import net.minecraft.server.EntityPlayer;
 import net.minecraft.server.PacketPlayOutHeldItemSlot;
+import net.minecraft.server.PacketPlayOutSetSlot;
 import net.minecraft.server.PlayerInventory;
 
 import org.apache.commons.lang.Validate;
@@ -32,6 +34,45 @@ public class CraftInventoryPlayer extends CraftInventory implements org.bukkit.i
         setItem(getHeldItemSlot(), stack);
     }
 
+    @Override
+    public void setItem(int index, ItemStack item) {
+        super.setItem(index, item);
+        EntityPlayer player = ((CraftPlayer) this.getHolder()).getHandle();
+        // PacketPlayOutSetSlot places the items differently than setItem()
+        //
+        // Between, and including, index 9 (the first index outside of the hotbar) and index 35 (the last index before
+        // armor slots) both PacketPlayOutSetSlot and setItem() places the items in the player's inventory the same way.
+        // Index 9 starts at the upper left corner of the inventory and moves to the right as it increases. When it
+        // reaches the end of the line it goes back to the left side of the new line in the inventory. Basically, it
+        // follows the path your eyes would follow as you read a book.
+        //
+        // The player's hotbar is indexed 0-8 in setItem(). The order goes: 0-8 hotbar, 9-35 normal inventory, 36 boots,
+        // 37 leggings, 38 chestplate, and 39 helmet. For indexes > 39 an ArrayIndexOutOfBoundsException will be thrown.
+        //
+        // PacketPlayOutSetSlot works very differently. Slots 0-8 are as follows: 0 crafting output, 1-4 crafting input,
+        // 5 helmet, 6 chestplate, 7 leggings, and 8 boots. Then, 9-35 work exactly the same as setItem(). The hotbar
+        // for PacketPlayOutSetSlot starts at index 36, and continues to index 44. Items placed where index is < 0 or
+        // > 44 have no action. Basically, the upper part of the player's inventory (crafting area and armor slots) is
+        // the first "row" of 9 slots for PacketPlayOutSetSlot. From there the rows work as normal, from left to right
+        // all the way down, including the hotbar.
+        //
+        // With this in mind, we have to modify the index we give PacketPlayOutSetSlot to match the index we intended
+        // with setItem(). First, if the index is 0-8, we need to add 36, or 4 rows worth of slots, to the index. This
+        // will push the item down to the correct spot in the hotbar.
+        //
+        // Now when index is > 35 (if index > 39 an ArrayIndexOutOfBoundsException will be thrown, so we need not worry
+        // about it) then we need to reset the index, and then count backwards  from the "top" of the inventory. That is
+        // to say, we first find (index - 36), which will give us the index required for the armor slots. Now, we need
+        // 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())
+            index = index + 36;
+        else if (index > 35)
+            index = 8 - (index - 36);
+        player.playerConnection.sendPacket(new PacketPlayOutSetSlot(player.activeContainer.windowId, index, CraftItemStack.asNMSCopy(item)));
+    }
+
     public int getHeldItemSlot() {
         return getInventory().itemInHandIndex;
     }