The tkinter doc could use a section on 'OS-tk peculiarities and interactions' ;-).  Anyway, thanks again.  Some of the 'Windows peculiarities' I noted above, where the test behaved better on Mac, may be due to the absence of calls that should be avoided on Mac.  Perhaps I can factor out some of the dialog setup and shutdown code, get it right for each platform, and then use it for all or at least multiple dialogs.