As you cant reproduce it, feel free to Pm me if you want me to take a look at the source.
We have located the issue, more to follow.
Thanks to Julian being patient, and a bunch of private messages, he identified what my program was doing that upset Windows.
Seems macOS handles some situation much differently, and is more tolerant (perhaps more than it should be)
Basically the issue boiled down to certain situations calling various Paint events too many times, in conjunction with what controls did or did not have a transparency flag…