What I expected to happen was:
A container being filled while dragging the mouse over the container slots while holding the pick block keybind

What actually happened was:
I was able to pick up a stack of items from my inventory but then dragging them and holding the pick block hotkey didn't put any new stacks of items into the container

Steps to Reproduce:

1. Change the keybind for 'pick block' to something other than the default 'Middle Button'
2. Make sure you are in creative mode
3. Prepare a container such as a chest in front of you with a stack of any item in your inventory
4. Open the container and hover over the stack of items with your mouse
5. Press the pick block keybind 
6. Drag your mouse to a slot within the container
7. Press and hold the pick block keybind while dragging the mouse over the slots of the container