Normally, in your inventory, if you have multiple partial stacks of items, you can double-click one, which will pick up that stack and fill it to the max stack size using other items in your inventory. For example, if you have five stacks of 20 cobblestone, double-clicking one will pick up a stack of 64, removing some from the other stacks.

In the new villager trading UI, this doesn't work. Double-clicking a stack simply picks up the partial stack, then places it back down in the inventory. This is frustrating in this case because trading often leaves you with partial stacks that you need to refill to continue trading.