Pick block by middle clicking (on_middleclick)
Use the middle mouse button (scrollwheel) on a block to pick it. If creative (this should be modded) it should always pick the block and place it in inventory and wield it, to quickly select a block that has been placed in the world.
In survival it would seek in your inventory if you have that block, and select/wield it.
In survival it would seek in your inventory if you have that block, and select/wield it.