Normally, you can double click an item to collect all of it from a ui, but doing so in the villager ui doesn't work. even for items you can't and aren't trying to trade with them.

1. Get a villager with a workstation
2. Give yourself 64 of something
3. put 1 in a separate slot and set the other 63 back down
4. double click one of the two partial stacks,

the stacks don't consolidate.