Doors have 2 independent hit-boxes.

A door is two-blocks tall and when breaking, it should be uniformly spread with in the 2-block sq. area and not reset the breaking animation and breaking of the door over all.
However in the recent beta doors have 2 independent hit-boxes within them one for the lower half and one for the top half. If we were to suppose break the bottom half and slowly move to the top half one the breaking animation (or just breaking the door) resets over-all.