OK, I’ve come up with a workaround:
I just started writing the event handler the way that I thought it should be written (utilizing both the old and new parameters), even though the new one wasn’t displaying in the parameter list, no matter how much cursing or begging I did. I just ignored this fact and wrote what I wanted to anyway.
As soon as I ran the code, the new parameter immediately appeared above the event hander. Problem solved.
Does that give you any hints?