Update

src/main/java/fr/themode/demo/PlayerInit.java

@@ -161,7 +161,7 @@ public class PlayerInit {
 
                 Inventory inventory = new Inventory(InventoryType.CHEST_1_ROW, "Test inventory");
                 inventory.setInventoryCondition((p, slot, inventoryConditionResult) -> {
-                    inventoryConditionResult.setCancel(true);
+                    inventoryConditionResult.setCancel(false);
                 });
                 inventory.setItemStack(0, item.clone());
 

src/main/java/fr/themode/minestom/event/PlayerSwapItemEvent.java

@@ -0,0 +1,30 @@
+package fr.themode.minestom.event;
+
+import fr.themode.minestom.item.ItemStack;
+
+public class PlayerSwapItemEvent extends CancellableEvent {
+
+    private ItemStack mainHandItem;
+    private ItemStack offHandItem;
+
+    public PlayerSwapItemEvent(ItemStack mainHandItem, ItemStack offHandItem) {
+        this.mainHandItem = mainHandItem;
+        this.offHandItem = offHandItem;
+    }
+
+    public ItemStack getMainHandItem() {
+        return mainHandItem;
+    }
+
+    public void setMainHandItem(ItemStack mainHandItem) {
+        this.mainHandItem = mainHandItem;
+    }
+
+    public ItemStack getOffHandItem() {
+        return offHandItem;
+    }
+
+    public void setOffHandItem(ItemStack offHandItem) {
+        this.offHandItem = offHandItem;
+    }
+}

src/main/java/fr/themode/minestom/inventory/Inventory.java

@@ -162,8 +162,6 @@ public class Inventory implements InventoryModifier, InventoryClickHandler, View
         if (clickResult.doRefresh())
             player.getPlayerConnection().sendPacket(getWindowItemsPacket());
 
-        System.out.println("cancel:" + clickResult.isCancel());
-
         if (isInWindow) {
             setItemStack(slot, clickResult.getClicked());
             setCursorPlayerItem(player, clickResult.getCursor());

src/main/java/fr/themode/minestom/inventory/PlayerInventory.java

@@ -66,14 +66,12 @@ public class PlayerInventory implements InventoryModifier, InventoryClickHandler
             for (int i = 0; i < items.length - 10; i++) {
                 ItemStack item = items[i];
                 StackingRule itemStackingRule = item.getStackingRule();
-                if (item.isAir()) {
-                    setItemStack(i, itemStack);
-                    return true;
-                } else if (itemStack.isSimilar(item)) {
-                    int itemAmount = item.getAmount();
+                if (itemStackingRule.canBeStacked(itemStack, item)) {
+                    int itemAmount = itemStackingRule.getAmount(item);
                     if (itemAmount == stackingRule.getMaxSize())
                         continue;
-                    int totalAmount = itemStack.getAmount() + itemAmount;
+                    int itemStackAmount = itemStackingRule.getAmount(itemStack);
+                    int totalAmount = itemStackAmount + itemAmount;
                     if (!stackingRule.canApply(itemStack, totalAmount)) {
                         item = itemStackingRule.apply(item, itemStackingRule.getMaxSize());
 
@@ -84,6 +82,9 @@ public class PlayerInventory implements InventoryModifier, InventoryClickHandler
                         sendSlotRefresh((short) convertToPacketSlot(i), item);
                         return true;
                     }
+                } else if (item.isAir()) {
+                    setItemStack(i, itemStack);
+                    return true;
                 }
             }
         }

src/main/java/fr/themode/minestom/inventory/click/InventoryClickProcessor.java

@@ -274,6 +274,7 @@ public class InventoryClickProcessor {
             cursor = result.getCursorItem();
             clicked = result.getClickedItem();
 
+            clickResult.setCancel(result.isCancel());
             if (result.isCancel()) {
                 clickResult.setClicked(clicked);
                 clickResult.setCursor(cursor);

src/main/java/fr/themode/minestom/listener/PlayerDiggingListener.java

@@ -5,6 +5,7 @@ import fr.themode.minestom.entity.ItemEntity;
 import fr.themode.minestom.entity.Player;
 import fr.themode.minestom.event.ItemDropEvent;
 import fr.themode.minestom.event.PlayerStartDiggingEvent;
+import fr.themode.minestom.event.PlayerSwapItemEvent;
 import fr.themode.minestom.instance.Instance;
 import fr.themode.minestom.instance.block.CustomBlock;
 import fr.themode.minestom.inventory.PlayerInventory;
@@ -88,8 +89,11 @@ public class PlayerDiggingListener {
                 PlayerInventory playerInventory = player.getInventory();
                 ItemStack mainHand = playerInventory.getItemInMainHand().clone();
                 ItemStack offHand = playerInventory.getItemInOffHand().clone();
-                playerInventory.setItemInMainHand(offHand);
-                playerInventory.setItemInOffHand(mainHand);
+                PlayerSwapItemEvent swapItemEvent = new PlayerSwapItemEvent(offHand, mainHand);
+                player.callCancellableEvent(PlayerSwapItemEvent.class, swapItemEvent, () -> {
+                    playerInventory.setItemInMainHand(swapItemEvent.getMainHandItem());
+                    playerInventory.setItemInOffHand(swapItemEvent.getOffHandItem());
+                });
                 break;
         }
     }

src/main/java/fr/themode/minestom/net/packet/client/handler/ClientPacketsHandler.java

@@ -15,7 +15,7 @@ public class ClientPacketsHandler {
     }
 
     public ClientPacket getPacketInstance(int id) {
-        // System.out.println("RECEIVED PACKET 0x" + Integer.toHexString(id));
+        //System.out.println("RECEIVED PACKET 0x" + Integer.toHexString(id));
         if (id > SIZE)
             throw new IllegalStateException("Packet ID 0x" + Integer.toHexString(id) + " has been tried to be parsed, debug needed");