I have my pick block key mapped to "r", and tried to middle click and drag to fill up slots with stacks of blocks, and discovered it did not work. I changed my pick block key back to default (middle click) and it worked.

How to reproduce:
1. Go to Options -> Controls -> Key Binds -> Pick Block and set it to a key
2. Go into a creative world, and pick up a block in your inventory
3. Middle click and drag in empty spots in your inventory

Expected result:
Your inventory gets filled with blocks

What happened:

Your inventory does not get filled with blocks