This behaviour is new to this release. Previous releases (many) have
been fine. However, I confess that I updated both Cygwin and XEmacs at
the same time so I don't know which update caused the breakage.
So, predictably, they have almost certainly broken it in cygwin. The special
select() handling in cygwin for /dev/mswindows is only used by XEmacs and I
suspect they have no tests for it.
andy