Richard, when replying by email, please strip quoted text except for an occasional line or two.  (See example of limited in-context quote below.)

A bug, for the purpose of this tracker, is a discrepancy between between the docs written by the PSF's core development team and the code provided by the same.  The PSF curses module is documented as unix-only.  In particular, "This extension module is designed to match the API of ncurses, an open-source curses library hosted on Linux and the BSD variants of Unix."  It does not run on the PSF (python.org) Windows distribution, because there is no C interface to a Windows implementation of curses.

>>> import curses  # 64-bit 3.6.1 on Win 10 using python.org installer
Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    import curses
  File "C:\Programs\Python36\lib\curses\__init__.py", line 13, in <module>
    from _curses import *
ModuleNotFoundError: No module named '_curses'

Anything Cygwin does to improve on this is their responsibility.

> how do you explain my success in running my wxPython emulation on all Cygwin releases since 2007

One or more people on the wxPython and/or Cygwin and/or other teams exerted the effort to make this happen.