Added CommandSender#isPlayer and CommandSender#isConsole

src/main/java/net/minestom/server/entity/Player.java

@@ -78,9 +78,13 @@ public class Player extends LivingEntity implements CommandSender {
     private PlayerSettings settings;
     private float exp;
     private int level;
+
     private PlayerInventory inventory;
-    private short heldSlot;
     private Inventory openInventory;
+    // Used internally to allow the closing of inventory within the inventory listener
+    private boolean didCloseInventory;
+
+    private short heldSlot;
 
     private Position respawnPoint;
 
@@ -1335,15 +1339,6 @@ public class Player extends LivingEntity implements CommandSender {
         }
     }
 
-    /**
-     * Get the player open inventory
-     *
-     * @return the currently open inventory, null if there is not (player inventory is not detected)
-     */
-    public Inventory getOpenInventory() {
-        return openInventory;
-    }
-
     /**
      * Used to get the {@link CustomBlock} that the player is currently mining
      *
@@ -1360,6 +1355,15 @@ public class Player extends LivingEntity implements CommandSender {
         return Collections.unmodifiableSet(bossBars);
     }
 
+    /**
+     * Get the player open inventory
+     *
+     * @return the currently open inventory, null if there is not (player inventory is not detected)
+     */
+    public Inventory getOpenInventory() {
+        return openInventory;
+    }
+
     /**
      * Open the specified Inventory, close the previous inventory if existing
      *
@@ -1394,7 +1398,7 @@ public class Player extends LivingEntity implements CommandSender {
 
     /**
      * Close the current inventory if there is any
-     * It closes the player inventory if {@link #getOpenInventory()} returns null
+     * It closes the player inventory (when opened) if {@link #getOpenInventory()} returns null
      */
     public void closeInventory() {
         Inventory openInventory = getOpenInventory();
@@ -1406,6 +1410,7 @@ public class Player extends LivingEntity implements CommandSender {
             getInventory().setCursorItem(ItemStack.getAirItem());
         } else {
             cursorItem = openInventory.getCursorItem(this);
+            openInventory.setCursorItem(this, ItemStack.getAirItem());
         }
         if (!cursorItem.isAir()) {
             // Add item to inventory if he hasn't been able to drop it
@@ -1424,6 +1429,30 @@ public class Player extends LivingEntity implements CommandSender {
         }
         playerConnection.sendPacket(closeWindowPacket);
         inventory.update();
+        this.didCloseInventory = true;
+    }
+
+    /**
+     * Used internally to prevent an inventory click to be processed
+     * when the inventory listeners closed the inventory
+     * <p>
+     * Should only be used within an inventory listener (event or condition)
+     *
+     * @return true if the inventory has been closed, false otherwise
+     */
+    public boolean didCloseInventory() {
+        return didCloseInventory;
+    }
+
+    /**
+     * Used internally to reset the didCloseInventory field
+     * <p>
+     * Shouldn't be used externally without proper understanding of its consequence
+     *
+     * @param didCloseInventory the new didCloseInventory field
+     */
+    public void UNSAFE_changeDidCloseInventory(boolean didCloseInventory) {
+        this.didCloseInventory = didCloseInventory;
     }
 
     /**

src/main/java/net/minestom/server/inventory/Inventory.java

@@ -186,14 +186,34 @@ public class Inventory implements InventoryModifier, InventoryClickHandler, View
      * Get the cursor item of a viewer
      *
      * @param player the player to get the cursor item from
-     * @return the player cursor item
+     * @return the player cursor item, air item if the player is not a viewer
-     * @throws IllegalStateException if {@code player} is not in the viewer list
      */
     public ItemStack getCursorItem(Player player) {
-        Check.stateCondition(!isViewer(player), "You can only get the cursor item of a viewer");
         return cursorPlayersItem.getOrDefault(player, ItemStack.getAirItem());
     }
 
+    /**
+     * Change the cursor item of a viewer,
+     * does nothing if {@param player} is not a viewer
+     *
+     * @param player     the player to change the cursor item
+     * @param cursorItem the new player cursor item
+     */
+    public void setCursorItem(Player player, ItemStack cursorItem) {
+        if (!isViewer(player))
+            return;
+
+        cursorItem = ItemStackUtils.notNull(cursorItem);
+
+        SetSlotPacket setSlotPacket = new SetSlotPacket();
+        setSlotPacket.windowId = -1;
+        setSlotPacket.slot = -1;
+        setSlotPacket.itemStack = cursorItem;
+        player.getPlayerConnection().sendPacket(setSlotPacket);
+
+        this.cursorPlayersItem.put(player, cursorItem);
+    }
+
     private synchronized void safeItemInsert(int slot, ItemStack itemStack) {
         itemStack = ItemStackUtils.notNull(itemStack);
         this.itemStacks[slot] = itemStack;

src/main/java/net/minestom/server/inventory/PlayerInventory.java

@@ -218,7 +218,13 @@ public class PlayerInventory implements InventoryModifier, InventoryClickHandler
      * @param cursorItem the new cursor item
      */
     public void setCursorItem(ItemStack cursorItem) {
-        this.cursorItem = ItemStackUtils.notNull(cursorItem);
+        cursorItem = ItemStackUtils.notNull(cursorItem);
+        this.cursorItem = cursorItem;
+        SetSlotPacket setSlotPacket = new SetSlotPacket();
+        setSlotPacket.windowId = -1;
+        setSlotPacket.slot = -1;
+        setSlotPacket.itemStack = cursorItem;
+        player.getPlayerConnection().sendPacket(setSlotPacket);
     }
 
     /**
@@ -233,7 +239,7 @@ public class PlayerInventory implements InventoryModifier, InventoryClickHandler
         Check.argCondition(!MathUtils.isBetween(slot, 0, getSize()),
                 "The slot " + slot + " does not exist for player");
         Check.notNull(itemStack, "The ItemStack cannot be null, you can set air instead");
-        
+
         EntityEquipmentPacket.Slot equipmentSlot;
 
         if (slot == player.getHeldSlot()) {

src/main/java/net/minestom/server/inventory/click/InventoryClickProcessor.java

@@ -532,6 +532,12 @@ public class InventoryClickProcessor {
                 }
             }
 
+            // Cancel the click if the inventory has been closed by Player#closeInventory within an inventory listener
+            if (player.didCloseInventory()) {
+                clickResult.setCancel(true);
+                player.UNSAFE_changeDidCloseInventory(false);
+            }
+
 
         }
         return clickResult;

src/main/java/net/minestom/server/listener/WindowListener.java

@@ -90,6 +90,10 @@ public class WindowListener {
         player.getPlayerConnection().sendPacket(windowConfirmationPacket);
     }
 
+    /**
+     * @param player    the player to refresh the cursor item
+     * @param inventory the player open inventory, null if not any (could be player inventory)
+     */
     private static void refreshCursorItem(Player player, Inventory inventory) {
         PlayerInventory playerInventory = player.getInventory();
 
@@ -100,8 +104,10 @@ public class WindowListener {
             cursorItem = playerInventory.getCursorItem();
         }
 
-        // Setting the window id properly seems to broke +64 stack support
+        // Error occurred while retrieving the cursor item, stop here
-        //byte windowId = inventory == null ? 0 : inventory.getWindowId();
+        if (cursorItem == null) {
+            return;
+        }
 
         SetSlotPacket setSlotPacket = new SetSlotPacket();
         setSlotPacket.windowId = -1;

src/main/java/net/minestom/server/utils/item/ItemStackUtils.java

@@ -5,6 +5,9 @@ import net.minestom.server.item.ItemStack;
 public class ItemStackUtils {
 
     /**
+     * Ensure that the returned ItemStack won't be null
+     * by replacing every null instance by a new Air one
+     *
      * @param itemStack the ItemStack to return if not null
      * @return {@code itemStack} if not null, {@link ItemStack#getAirItem()} otherwise
      */