Just farming a lot of expirience using furnace. Steps to reproduce:
Place furnace on top of the hopper.
Smelt something (i.e. iron ore)
Now, just spam-click at output slot in furnace (pick item before hopper get it), + it needed to move the maximum ammount of that item from your inventory to your clicked item (like does double-click on item usually).
Done, here is your expirience from [count of items on mouse] items.
Cleaning up old tickets: This ticket has not been updated recently (~1 year+) so is being closed as Cannot Reproduce. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines.
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