In the iron or diamond chisel interface, when a block is set to be chiseled in the upper left, hovering the mouse over one of the possible chisel styles and hitting the "Drop Item" button will both drop a copy of that chiseled block and pick up the stack from the upper left with the cursor without reducing the number of items in the stack. This also happens if one of the item slot hotkeys is pressed and the slot is open; the stack will be picked up and an extra item will be placed in the slot.

Tested and confirmed this happens in a fresh 1.12.2 installation without InventoryTweaks, MouseTweaks, or JEI.