If I was to hold a wooden trapdoor in my main hand, I could hold my sneak key and then place it onto the side of a crafting table. But, if I was to hold a wooden trapdoor in my offhand and nothing in my main hand, I would hold my sneak key and open up the crafting table GUI. It appears as though sneak placement hasn't been incorporated to the offhand, so I hope that this is fixed in the near future.