Fix/legacy drop slot desync

common/src/main/java/com/viaversion/viabackwards/protocol/v1_13_1to1_13/Protocol1_13_1To1_13.java

@@ -25,6 +25,7 @@
 import com.viaversion.viabackwards.protocol.v1_13_1to1_13.rewriter.EntityPacketRewriter1_13_1;
 import com.viaversion.viabackwards.protocol.v1_13_1to1_13.rewriter.ItemPacketRewriter1_13_1;
 import com.viaversion.viabackwards.protocol.v1_13_1to1_13.rewriter.WorldPacketRewriter1_13_1;
+import com.viaversion.viabackwards.protocol.v1_13_1to1_13.storage.PlayerInventoryState;
 import com.viaversion.viaversion.api.connection.UserConnection;
 import com.viaversion.viaversion.api.minecraft.ClientWorld;
 import com.viaversion.viaversion.api.minecraft.RegistryType;
@@ -33,6 +34,7 @@
 import com.viaversion.viaversion.api.protocol.packet.PacketWrapper;
 import com.viaversion.viaversion.api.protocol.remapper.PacketHandlers;
 import com.viaversion.viaversion.api.protocol.remapper.ValueTransformer;
+import com.viaversion.viaversion.api.protocol.version.ProtocolVersion;
 import com.viaversion.viaversion.api.type.Types;
 import com.viaversion.viaversion.data.entity.EntityTrackerBase;
 import com.viaversion.viaversion.libs.gson.JsonElement;
@@ -92,6 +94,7 @@ public String transform(PacketWrapper wrapper, String inputValue) {
             public void register() {
                 map(Types.UNSIGNED_BYTE); // Id
                 map(Types.STRING); // Window Type
+                handler(wrapper -> wrapper.user().get(PlayerInventoryState.class).setOpenContainerId(wrapper.get(Types.UNSIGNED_BYTE, 0)));
                 handler(wrapper -> {
                     JsonElement title = wrapper.passthrough(Types.COMPONENT);
                     translatableRewriter.processText(wrapper.user(), title);
@@ -112,6 +115,50 @@ public void register() {
             }
         });
 
+        registerClientbound(ClientboundPackets1_13.CONTAINER_CLOSE, wrapper -> {
+            wrapper.passthrough(Types.UNSIGNED_BYTE); // Id
+            wrapper.user().get(PlayerInventoryState.class).setOpenContainerId(0);
+        });
+        registerServerbound(ServerboundPackets1_13.CONTAINER_CLOSE, wrapper -> {
+            wrapper.passthrough(Types.BYTE); // Id
+            wrapper.user().get(PlayerInventoryState.class).setOpenContainerId(0);
+        });
+        registerClientbound(ClientboundPackets1_13.SET_CARRIED_ITEM, wrapper -> {
+            wrapper.user().get(PlayerInventoryState.class).setSelectedHotbarSlot(wrapper.passthrough(Types.BYTE));
+        });
+        registerServerbound(ServerboundPackets1_13.SET_CARRIED_ITEM, wrapper -> {
+            wrapper.user().get(PlayerInventoryState.class).setSelectedHotbarSlot(wrapper.passthrough(Types.SHORT));
+        });
+        registerServerbound(ServerboundPackets1_13.PLAYER_ACTION, wrapper -> {
+            final int status = wrapper.passthrough(Types.VAR_INT);
+            if (status != 3 && status != 4) { // Drop stack / drop item
+                return;
+            }
+
+            // 1.17.1+ servers skip the hotbar slot resync after a drop action, assuming the client predicted
+            // it (ServerPlayer#drop marks the remote slot as already known) - clients only predict drops since
+            // 1.13.1, so on older clients the dropped stack ghosts in the hotbar. Replay the drop as the
+            // equivalent throw-click instead: the click path does get resynced. Clients only send drop actions
+            // with no screen open; if the server still has one open, the click would miss - leave the vanilla
+            // action alone there, as on older servers, which resync drops themselves.
+            final boolean suppressingServer = wrapper.user().getProtocolInfo().serverProtocolVersion()
+                .newerThanOrEqualTo(ProtocolVersion.v1_17_1);
+            final PlayerInventoryState state = wrapper.user().get(PlayerInventoryState.class);
+            if (!suppressingServer || state.openContainerId() != 0) {
+                return;
+            }
+
+            wrapper.cancel();
+            final PacketWrapper click = wrapper.create(ServerboundPackets1_13.CONTAINER_CLICK);
+            click.write(Types.BYTE, (byte) 0); // Player inventory
+            click.write(Types.SHORT, (short) (36 + state.selectedHotbarSlot()));
+            click.write(Types.BYTE, (byte) (status == 3 ? 1 : 0)); // Throw button: 1 = full stack, 0 = single
+            click.write(Types.SHORT, (short) 0); // Action id the client never used; its confirmation is ignored
+            click.write(Types.VAR_INT, 4); // Throw
+            click.write(Types.ITEM1_13, null); // Unknown content; a mismatch only makes the server resync
+            click.sendToServer(Protocol1_13_1To1_13.class);
+        });
+
         registerClientbound(ClientboundPackets1_13.COMMAND_SUGGESTIONS, new PacketHandlers() {
             @Override
             public void register() {
@@ -161,6 +208,7 @@ public void register() {
     public void init(UserConnection user) {
         user.addEntityTracker(getClass(), new EntityTrackerBase(user, EntityTypes1_13.EntityType.PLAYER));
         user.addClientWorld(getClass(), new ClientWorld());
+        user.put(new PlayerInventoryState());
     }
 
     @Override

common/src/main/java/com/viaversion/viabackwards/protocol/v1_13_1to1_13/storage/PlayerInventoryState.java

@@ -0,0 +1,42 @@
+/*
+ * This file is part of ViaBackwards - https://github.com/ViaVersion/ViaBackwards
+ * Copyright (C) 2016-2026 ViaVersion and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package com.viaversion.viabackwards.protocol.v1_13_1to1_13.storage;
+
+import com.viaversion.viaversion.api.connection.StorableObject;
+
+public final class PlayerInventoryState implements StorableObject {
+
+    private short selectedHotbarSlot = 0;
+    private int openContainerId = 0; // 0 = none open (the server never assigns 0 to a screen)
+
+    public short selectedHotbarSlot() {
+        return selectedHotbarSlot;
+    }
+
+    public void setSelectedHotbarSlot(final short slot) {
+        this.selectedHotbarSlot = slot;
+    }
+
+    public int openContainerId() {
+        return openContainerId;
+    }
+
+    public void setOpenContainerId(final int containerId) {
+        this.openContainerId = containerId;
+    }
+}