fix an issue where opening a box that is stacked on top of another bo…

…x would make the game's UI think the player obtained both the contents of the top box and the bottom box

src/game/client/swarm/gameui/swarm/vitemshowcase.cpp

@@ -441,15 +441,32 @@ void ItemShowcase::ShowItems( SteamInventoryResult_t hResult, int iStart, int iC
 			delete pInstance;
 			continue;
 		}
+
 		const ReactiveDropInventory::ItemDef_t *pDef = ReactiveDropInventory::GetItemDef( pInstance->ItemDefID );
 		if ( !pDef || pDef->GameOnly )
 		{
 			delete pInstance;
 			continue;
 		}
 
+		if ( const ReactiveDropInventory::ItemInstance_t *pPrev = ReactiveDropInventory::GetLocalItemCache( pInstance->ItemID ) )
+		{
+			if ( pPrev->Quantity > pInstance->Quantity )
+			{
+				// we consumed items from a stack
+				delete pInstance;
+				continue;
+			}
+			else if ( pPrev->Quantity < pInstance->Quantity && pDef->AutoStack )
+			{
+				// we added items to a stack
+				pInstance->Quantity -= pPrev->Quantity;
+			}
+		}
+
 		if ( pDef->AutoStack )
 		{
+			// replace previous versions of this item's state in the queue
 			bool bFound = false;
 			FOR_EACH_VEC( pItemShowcase->m_Queue, i )
 			{