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Fix inventory desync for <1.17 clients on 1.17+ servers #1243

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Merged
kennytv merged 1 commit into from
Apr 10, 2026
+32 −3
Merged

Fix inventory desync for <1.17 clients on 1.17+ servers #1243

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12 changes: 11 additions & 1 deletion ...rc/main/java/com/viaversion/viabackwards/protocol/v1_17_1to1_17/Protocol1_17_1To1_17.java
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
Expand All @@ -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);
}
});

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23 changes: 21 additions & 2 deletions .../viaversion/viabackwards/protocol/v1_17to1_16_4/rewriter/BlockItemPacketRewriter1_17.java
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -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) {
Expand Down