Can it rest on the shingles when the ladder being used vertically?

There is not a problem using the AC78 on shingles on a roof as long as the ladder is set at the proper 4/1 angle, the locks are locked, and the feet are set on firm level ground as per the instructions on the ladder. If the application does not allow for all of those set-up requirements for an extension ladder then it would be an improper use to put the stabilizer on to the roof.