Thanks, Leland. Here's the sit:
A 12-2 NM brings power into a s-p switch. Another 12-2 NM leaves the switchbox as a switchleg to the light fixture. (The two neutrals in the switchbox are spliced together.)

At the 4-0 box for the light fixture (but without the fixture yet installed), the black and the white both from the switchleg are wirenutted together. Why no short? Wouldn't the neutral provide a path back to the breaker?

(Is this gonna be one of those answers where I slap my head and say "OMG, what a dufus I am!"?