Fix inventory desync for <1.17 clients on 1.17+ servers

This addresses the known issue where <1.17 clients experience inventory
desyncs on certain inventory click actions when connected to 1.17+ servers.

The root cause is threefold:

1. Carried item not forwarded from CONTAINER_SET_CONTENT (1.17.1->1.17)

   In 1.17.1+, CONTAINER_SET_CONTENT includes a carried/cursor item
   field that doesn't exist in <1.17 formats. The handler stripped
   this field and stored it internally, but never forwarded it to the
   client. This caused the cursor to remain desynced after cancelled
   clicks.

   Fix: Send the carried item as a separate CONTAINER_SET_SLOT packet
   (windowId=-1, slot=-1) so the client's cursor state is properly
   synced.

2. Empty changedSlots array in CONTAINER_CLICK translation (1.17->1.16.4)

   The serverbound CONTAINER_CLICK was translated with an empty
   changedSlots array. The server uses this to detect client-server
   desync and send corrections. With an empty array, the server
   skipped sending corrections for cancelled InventoryClickEvents.

   Fix: For PICKUP mode (mode 0), include the clicked slot with a
   null predicted value so the server detects the desync and sends
   a SET_SLOT correction.

   For non-PICKUP modes (shift-click, swap, throw, drag, etc.)
   which affect multiple unpredictable slots, force a state ID
   mismatch to trigger a full CONTAINER_SET_CONTENT resync.

Author: JobsonMarinho

common/src/main/java/com/viaversion/viabackwards/protocol/v1_17_1to1_17/Protocol1_17_1To1_17.java

@@ -71,7 +71,7 @@ protected void registerPackets() {
             // Length is encoded as a var int in 1.17.1
             wrapper.write(Types.ITEM1_13_2_SHORT_ARRAY, wrapper.read(Types.ITEM1_13_2_ARRAY));
 
-            // Carried item - should work without adding it to the array above
+            // Carried item - forward as CONTAINER_SET_SLOT for <1.17 clients
             Item carried = wrapper.read(Types.ITEM1_13_2);
 
             PlayerLastCursorItem lastCursorItem = wrapper.user().get(PlayerLastCursorItem.class);
@@ -82,6 +82,16 @@ protected void registerPackets() {
                 // for a subsequent drag
 
                 lastCursorItem.setLastCursorItem(carried);
+
+                // In 1.17.1+, the carried/cursor item is part of CONTAINER_SET_CONTENT,
+                // but <1.17 clients don't have this field. Send it as a separate
+                // CONTAINER_SET_SLOT packet so the client's cursor state is properly synced.
+                // This fixes inventory desyncs when the server cancels InventoryClickEvents.
+                PacketWrapper cursorPacket = wrapper.create(ClientboundPackets1_17.CONTAINER_SET_SLOT);
+                cursorPacket.write(Types.BYTE, (byte) -1); // Window ID: -1 for cursor
+                cursorPacket.write(Types.SHORT, (short) -1); // Slot: -1 for cursor
+                cursorPacket.write(Types.ITEM1_13_2, carried);
+                cursorPacket.send(Protocol1_17_1To1_17.class);
             }
         });
 

common/src/main/java/com/viaversion/viabackwards/protocol/v1_17to1_16_4/rewriter/BlockItemPacketRewriter1_17.java

@@ -26,6 +26,7 @@
 import com.viaversion.viabackwards.api.rewriters.MapColorRewriter;
 import com.viaversion.viabackwards.protocol.v1_17to1_16_4.Protocol1_17To1_16_4;
 import com.viaversion.viabackwards.protocol.v1_17to1_16_4.data.MapColorMappings1_16_4;
+import com.viaversion.viabackwards.protocol.v1_17_1to1_17.storage.InventoryStateIds;
 import com.viaversion.viabackwards.protocol.v1_17to1_16_4.storage.PlayerLastCursorItem;
 import com.viaversion.viaversion.api.data.entity.EntityTracker;
 import com.viaversion.viaversion.api.minecraft.BlockChangeRecord;
@@ -83,8 +84,26 @@ public void register() {
                     int mode = wrapper.passthrough(Types.VAR_INT); // Mode
                     Item clicked = handleItemToServer(wrapper.user(), wrapper.read(Types.ITEM1_13_2)); // Clicked item
 
-                    // The 1.17 client would check the entire inventory for changes before -> after a click and send the changed slots here
-                    wrapper.write(Types.VAR_INT, 0); // Empty array of slot+item
+                    // The 1.17 client would check the entire inventory for changes before -> after a click
+                    // and send the changed slots here. We include the clicked slot so the server
+                    // detects any desync (e.g. cancelled InventoryClickEvent) and sends corrections.
+                    if (slot >= 0 && clicked != null) {
+                        wrapper.write(Types.VAR_INT, 1); // One modified slot
+                        wrapper.write(Types.SHORT, slot); // The clicked slot
+                        wrapper.write(Types.ITEM1_13_2, (Item) null); // Predicted: empty (item picked up)
+                    } else {
+                        wrapper.write(Types.VAR_INT, 0); // Empty array of slot+item
+                    }
+
+                    // Non-PICKUP modes (shift-click, swap, throw, drag, etc.) affect multiple slots
+                    // that we can't easily predict. Force a state ID mismatch so the server
+                    // sends a full inventory resync instead of individual slot corrections.
+                    if (mode != 0) {
+                        InventoryStateIds stateIds = wrapper.user().get(InventoryStateIds.class);
+                        if (stateIds != null) {
+                            stateIds.setStateId(wrapper.get(Types.BYTE, 0), -1);
+                        }
+                    }
 
                     PlayerLastCursorItem state = wrapper.user().get(PlayerLastCursorItem.class);
                     if (mode == 0 && button == 0 && clicked != null) {