Fix: Shelf block entity items rendered in wrong slot for Bedrock clients (#6225)

* Fix: Shelf block entity items rendered in wrong slot for Bedrock clients

Bedrock determines shelf item position by list index rather than
a Slot tag, so a sparse Items list causes items to shift left into
the wrong visual position. This produces a dense 3-element list
with empty entries for unoccupied slots.

* Move static final EMPTY_ITEM variable to BedrockItemBuilder, add comment with last tested note

---------

Co-authored-by: onebeastchris <github@onechris.mozmail.com>

Author: LPettay

core/src/main/java/org/geysermc/geyser/translator/item/BedrockItemBuilder.java

@@ -41,6 +41,9 @@
  * An intermediary class made to allow easy access to work-in-progress NBT, such as lore and display.
  */
 public final class BedrockItemBuilder {
+
+    public static final NbtMap EMPTY_ITEM = BedrockItemBuilder.createItemNbt("", 0, 0).build();
+
     // All Bedrock-style
     @Nullable
     private String customName;

core/src/main/java/org/geysermc/geyser/translator/level/block/entity/ShelfBlockEntityTranslator.java

@@ -31,19 +31,35 @@
 import org.geysermc.geyser.item.parser.ItemStackParser;
 import org.geysermc.geyser.level.block.type.BlockState;
 import org.geysermc.geyser.session.GeyserSession;
+import org.geysermc.geyser.translator.item.BedrockItemBuilder;
 import org.geysermc.mcprotocollib.protocol.data.game.level.block.BlockEntityType;
 
-import java.util.Comparator;
+import java.util.Arrays;
+import java.util.List;
 
 @BlockEntity(type = BlockEntityType.SHELF)
 public class ShelfBlockEntityTranslator extends BlockEntityTranslator {
 
+    private static final int SLOT_COUNT = 3;
+
     @Override
     public void translateTag(GeyserSession session, NbtMapBuilder bedrockNbt, NbtMap javaNbt, BlockState blockState) {
         // We can't translate align_items_to_bottom, I think :(
-        bedrockNbt.putList("Items", NbtType.COMPOUND, javaNbt.getList("Items", NbtType.COMPOUND).stream()
-            .sorted(Comparator.comparingInt(stack -> stack.getByte("Slot")))
-            .map(stack -> ItemStackParser.javaItemStackToBedrock(session, stack).build())
-            .toList());
+
+        // Bedrock determines shelf item position by list index, so we must produce
+        // a dense list with empty items for unoccupied slots.
+        // Verified with BDS 1.26.0.2
+        NbtMap[] items = new NbtMap[SLOT_COUNT];
+        Arrays.fill(items, BedrockItemBuilder.EMPTY_ITEM);
+
+        List<NbtMap> javaItems = javaNbt.getList("Items", NbtType.COMPOUND);
+        for (NbtMap stack : javaItems) {
+            int slot = stack.getByte("Slot");
+            if (slot >= 0 && slot < SLOT_COUNT) {
+                items[slot] = ItemStackParser.javaItemStackToBedrock(session, stack).build();
+            }
+        }
+
+        bedrockNbt.putList("Items", NbtType.COMPOUND, Arrays.asList(items));
     }
 }