Items with max stack amount can be stacked up to 64 in the new villager trades menu.

1. Summon any Villager that can buy non-stackable item or item with a max stack amount less than 64 (e.g. minecraft:oak_boat)

   /summon villager ~ ~1 ~ {NoAI:1,Offers:{Recipes:[{buy:{id:oak_boat,Count:1},sell:{id:emerald,Count:1}}]}}
   

2. Give yourself more than one oak_boat.
3. Open trade menu, click on the trade which you modified before.
4. You can see there is more than one boat in the first trade item slot. Now take this item stack and place it in your inventory. It kept its amount.

You can also summon a Fisherman, upgrade him to a Master level and - if you got a trade which takes a boat - you can get item stacks over their max amount, but limited to 64.

How it should work:

It should place only max amount of this item in a trade slot instead of 64,
AND ONE OF:
After clicking an item stack with amount > max amount you should get only its max amount on cursor,
OR
After placing such item stack in any inventory it should place in that inventory only max amount of that item.