Placing an incomplete stack into the input or fuel of the furnace draws units quickly from other stacks in your inventory after the incomplete stack has been transferred. I expected the transfer of units to behave the same way as transferring units in a chest or any other block UI - the transfer stops after the incomplete stack has been exhausted even if I kept my finger pressed (or mouse clicked) to continue transferring.

To reproduce this bug, you need at least two stacks of the same item in your inventory, one of which is incomplete. For example, two stacks of spruce tree blocks (64 and 30 lets say) for the input slot. This bug happens identically for stacks of items that go into the fuel slot.

Open the furnace and long press/click the incomplete stack. As expected, the green bar will fill horizontally. Keep pressing/clicking after the green bar is complete and you will see units from the number of units in the input cell very rapidly increase to 64 and the number of units in the other stack decrease accordingly. This was unexpected.

I expected to be able to drop stacks of items into the input and fuel slots of the furnace the same way that I drop stacks of items in the chest and elsewhere. Continuing to hold the press/click should not change anything.

Devices tested:
Samsung Galaxy S4 Android 5.0.1 MCPE 0.12.3 (tested touch input)
Asus B121 Tablet Win 10 Pro MCPE 0.12.3 (tested both touch and keyboard/mouse input)