What I expected was:
To move an item from a stack in inventory to the first slot of the Anvil.
What happened instead:
One item went into the first slot, and the stack from which it came was consumed / vanished / eaten.
Steps to reproduce:
1) Put a stack of, for example, carrots in an inventory slot, for example, slot 8.
2) Click the anvil to start using it.
3) Mouse over the first anvil slot, intending to put the carrot(s) there, and so press the '8' key.
In other interfaces, this would swap the contents of that hovered slot, with the contents of inventory slot 8 – specifically here, you would expect the bunch of carrots to go over. Or maybe just one would go over, and the rest would remain.
4) But instead, you will see, one goes to the anvil, and the rest of the carrots have absconded.