Thanks for doing all the testing, Ned.  I guess that the path forward is now clear.  I will revert to the 8.6.11 code for 10.15 and earlier and use the new code for 11 and later.  Of course the 8.6.11 code already has two cases, for 10.14 and earlier and 10.15 and later.  So I will have some code cleanup to do.

The UFO, by the way, was a standalone file dialog which was first being rendered as a separate window and then immediately attached to the parent window as a sheet.  This was easier to see on a slower machine, like a VMWare VM which does not have graphics acceleration.

Evidently the big change for 10.15 was that the file dialog was changed so that it became managed by a separate process, rather than just borrowing the current process's event queue.  The reason, supposedly, was to make it so that an app running in a sandbox could still read and write the user's files.  It seems that Apple had a hard time making this work, and the UFO as well as the assertion error suggest to me that they still haven't figured it out.  So I thnk we can expect more trouble with the next OS.  At least we have a year before that adventure starts.

Needless to say, having some documentation from Apple would be much more pleasant than the trial and error method that we are forced to use now.  But we know better than to expect that.