--- a/net/minecraft/world/inventory/ContainerSynchronizer.java
+++ b/net/minecraft/world/inventory/ContainerSynchronizer.java
@@ -11,4 +_,6 @@
     void sendCarriedChange(AbstractContainerMenu containerMenu, ItemStack stack);
 
     void sendDataChange(AbstractContainerMenu container, int id, int value);
+
+    default void sendOffHandSlotChange() {} // Paper - Sync offhand slot in menus
 }
