Add data components hash code to translated NBT

core/src/main/java/org/geysermc/geyser/item/type/Item.java

@@ -151,6 +151,10 @@ public class Item {
         if (repairCost != null) {
             builder.putInt("RepairCost", repairCost);
         }
+
+        // Prevents the client from trying to stack items with untranslated components
+        // Relies on correct hash code implementation, and some luck
+        builder.putInt("GeyserHash", components.hashCode()); // TODO: don't rely on this
     }
 
     /**

core/src/main/java/org/geysermc/geyser/translator/inventory/PlayerInventoryTranslator.java

@@ -261,6 +261,8 @@ public class PlayerInventoryTranslator extends InventoryTranslator {
                         GeyserItemStack sourceItem = inventory.getItem(sourceSlot);
                         if (playerInv.getCursor().isEmpty()) {
                             playerInv.setCursor(sourceItem.copy(0), session);
+                        } else if (!InventoryUtils.canStack(sourceItem, playerInv.getCursor())) {
+                            return rejectRequest(request);
                         }
 
                         playerInv.getCursor().add(transferAmount);
@@ -272,6 +274,8 @@ public class PlayerInventoryTranslator extends InventoryTranslator {
                         GeyserItemStack sourceItem = playerInv.getCursor();
                         if (inventory.getItem(destSlot).isEmpty()) {
                             inventory.setItem(destSlot, sourceItem.copy(0), session);
+                        } else if (!InventoryUtils.canStack(sourceItem, inventory.getItem(destSlot))) {
+                            return rejectRequest(request);
                         }
 
                         inventory.getItem(destSlot).add(transferAmount);
@@ -284,6 +288,8 @@ public class PlayerInventoryTranslator extends InventoryTranslator {
                         GeyserItemStack sourceItem = inventory.getItem(sourceSlot);
                         if (inventory.getItem(destSlot).isEmpty()) {
                             inventory.setItem(destSlot, sourceItem.copy(0), session);
+                        } else if (!InventoryUtils.canStack(sourceItem, inventory.getItem(destSlot))) {
+                            return rejectRequest(request);
                         }
 
                         inventory.getItem(destSlot).add(transferAmount);

core/src/main/java/org/geysermc/geyser/util/InventoryUtils.java

@@ -178,11 +178,26 @@ public class InventoryUtils {
     }
 
     public static boolean canStack(GeyserItemStack item1, GeyserItemStack item2) {
+        if (GeyserImpl.getInstance().getConfig().isDebugMode())
+            canStackDebug(item1, item2);
         if (item1.isEmpty() || item2.isEmpty())
             return false;
         return item1.getJavaId() == item2.getJavaId() && Objects.equals(item1.getComponents(), item2.getComponents());
     }
 
+    private static void canStackDebug(GeyserItemStack item1, GeyserItemStack item2) {
+        DataComponents components1 = item1.getComponents();
+        DataComponents components2 = item2.getComponents();
+        if (components1 != null && components2 != null) {
+            if (components1.hashCode() == components2.hashCode() && !components1.equals(components2)) {
+                GeyserImpl.getInstance().getLogger().error("DEBUG: DataComponents hash collision");
+                GeyserImpl.getInstance().getLogger().error("hash: " + components1.hashCode());
+                GeyserImpl.getInstance().getLogger().error("components1: " + components1);
+                GeyserImpl.getInstance().getLogger().error("components2: " + components2);
+            }
+        }
+    }
+
     /**
      * Checks to see if an item stack represents air or has no count.
      */