The trigger posted above does not work. Just tested it.
But, as an aside, you might run into problems if you plan on using this in a map where Zerg drones mutate into buildings. As you know, 1 Drone has -1 Supply... but, an Extractor or a Spawning pool has 0 supply (both finished, and when in construction). Cancel a building during construction and your supply goes -1, then +1. Could be an issue if a drone is your 1/0 supply situation.
The trigger posted above does not work. Just tested it.
But, as an aside, you might run into problems if you plan on using this in a map where Zerg drones mutate into buildings. As you know, 1 Drone has -1 Supply... but, an Extractor or a Spawning pool has 0 supply (both finished, and when in construction). Cancel a building during construction and your supply goes -1, then +1. Could be an issue if a drone is your 1/0 supply situation.