What I expected was...:
When using the Pick Block key on an item in the top-left corner of a container, it should be duplicated, just like any other item in the container.
What actually happened was...:
Nothing - the item was not duplicated.
Steps to reproduce:
1. Make sure you are in Creative mode.
2. Place an item at the top-left corner of any container (chest, doublechest, dispenser, furnace, anvil, etc.).
3. Press the Pick Block key (middle mouse button by default) on the item. Observe that nothing happens.
4. Move the item to another slot and press the Pick Block key on it again. Observe that it is duplicated.