EDIT: Immediately after creating this I realized that this is actually invalid; this functionality is simply the result of the spacebar reselecting the already-selected trade.

When a non-full item stack exists in a villager's trade input slot, it is possible to refill it with items from the player's inventory by pressing the spacebar. The key binds screen does not give any indication of this behavior, much less offer an option to reconfigure it.

This issue can be compared to MC-147718 and its related issues. Among this set of issues, the more glaring ones have been considered valid and the more niche ones have been considered feature requests. I would not be surprised if this issue is treated as the latter; I'm mainly reporting it to ensure that both Mojang and the community can more easily understand the scope and history of the larger non-configurable-controls problem area.