Hi Paul,
No solution from me, I fear. This was one of the first things I tried with M1, but failed miserably and went for a work-around. Have avoided combo-boxes like a plague since then.
Watching this thread carefully, hoping to learn something.
As an aside, as I do not use combos currently, it is somewhat annoying to find my Apps bulked with all the spinner stuff. Is it possible to eliminate them.