Pressing Enter while hovering over the output field allows you to trade with a villager that is out of stock

Steps to reproduce:

1) Grab any villager with enough items to clear out its stock

2) Select a trade

3) Shift+click the output box to get the items, then press enter to add more items to the input

4) Repeat step 4 until the villager is out of stock

5) Press enter on the output box again, then shift+click. You should still be able to trade

If that confuses you, I have attached a video to help