wish8.6 perfect in Cygwin32, misbehaves in Cygwin64

[Fergus Daly]

Can't entirely recall* when wish8.4 was versioned up to the current wish8.6 in Cygwin,
or whether there was an intermediate wish8.5 - I think it must be nearly a decade ago, or possibly
longer - but I do remember that
1. wish8.4 "worked" in bash and rxvt terminals (I think all this preceded mintty) AND also in xterm
2. wish8.[56] "fails" in bash and mintty terminals and insists on DISPLAY being set, i.e. requires XWin
(by which I mean wish was the driver for Tcl/Tk menus, windows, other displays).
Actually I hung on to the components that comprise wish8.4 and toggle between them at will and can
confirm  the truth of (1) and (2) above. 
Generally though, I guess users have upgraded to wish8.6 long since and if they use it to provide Tcl/Tk
menus (?) or sidebar Help Tcl/Tk windows (?) are used to requiring e.g. xterm in XWin in order to do so.  

ALL THIS in Cygwin-32.

Recent unconnected threads in this list have persuaded me to trial Cygwin-64 (and, if happy, I'll jettison the
Cygwin-32 platform). I have installed it identically (see setup -P below).
The many contexts in which wish8.6 has worked properly in Cygwin-32 are, for me, failing uniformly in
Cygwin-64. By this I mean, Tcl/Tk menus are mutely failing to appear, Tcl/Tk Help windows are mutely failing
to appear.
Can't think what I'm getting wrong. But a good first step would be: have any of you, in advancing from Cygwin-32
to what was intended to provide identical functionality in Cygwin-64, experienced similar unexplained glitches,
and managed to solve them? Or is this experience unique? If you can help with this specific Tcl/Tk problem, that
would be perfect!
BTW I use
setup-x86[_64] -P ..,..,tcl-tix,tcl-tk-devel,..,.. 
to build the supporting platforms.
Thank you!

* "Can't entirely recall .. .." (line 1):
Typing "site:cygwin.com searchstring" into the Google search bar used to yield pages of output, including hits for
very specific strings e.g. "site:cygwin.com fergus grap" to remind myself what I asked about grap as long ago as
2001 - and, much more helpfully, what somebody kindly answered.  Now, after a measly trickle of hits, Google
explains the paucity with a reference to data protection. I know I asked specifically about the loss of versatility
with wish8.4 at the time, but cannot find the thread. 
Is there a vehicle which still permits an exhaustive search of the entire Cygwin project mailing archive?