The bug
If holding down your swap hands key (default "f") over the output slot of a furnace, when the furnace finishes cooking whatever it was cooking, occasionally, no item will show up in the offhand. This is not actually true, because if you click the swap hands key again over an empty slot in your inventory, the item from your offhand will show itself.
How to reproduce
Put fuel and a smeltable item in a furnace
As the item is getting closer to being smelted, hold in swap hands key over output of furnace
❌ Occasionally, notice that no item shows in offhand
Linked issues
Attachments
Comments 2
This has changed in 21w10a. Previously, the number of items in the furnace slot would not update until the item that was brought into the offhand was updated. Now, the number of items updates immediately in the furnace, despite the item brought into the offhand not showing. Additionally, the player previously needed to click once on the empty slot where the offhand item was brought for it to reveal itself. Now, they do not have to complete this step to reproduce this issue.
Same thing happens when you speedily, repeatedly shift click on the output until the furnace has smelted one item. The item disappeares and is invisible in your inventory until you update it by taking out the item in the top furnace slot (which still renders even though it is gone), or clicking the place where the item is supposed to be (or the stack that it's number is not appearing on).