Wrong trade amount calculation on Villager/NPCs when using shift click

If you try to reproduce the bug from https://mojang.atlassian.net/browse/MC-1580 and you look closely, you will also find that you got one "offer" to many out of your sold items.

How to reproduce it:
Pick a stack (64) of diamonds and put it into slot 1. Shift-click (this is the important part) the emeralds in slot3 and you get 12 emeralds leaving 4 diamonds in slot1.
Since the max amount to buy per recipe is 7, you just found the visual bug from #MC-1580 and may now try to pick up the 12 emeralds, which will now update trade to the correct max amount.
So you get 29 diamonds (64 - 7x5 diamonds) in slot 1 now, but look... you just bought 8 emeralds instead of just 7

My guess is, that the update/correction of the UI bug does not obeye(subtract) the one trade item which would stay in slot3 when buying one item after another without using shift click.