XDMCP connection hangs, then blanks, after a while

[Henry S. Thompson]

I've enabled XDMCP on a local (debian/lightdm) box, and can
successfully connect to it from Cygwin using a modified version of
startxwin in order to launch

  /usr/bin/XWin :1 -clipboard -dpi 100 -query [my host] -auth [me]/.serverauth.6351

All works well, useful work is done, go and have a coffee, come back
and it's frozen (cursor follows mouse, but no kbd or click has any
effect).

Leave it a while longer, and it goes black.  I _think_ the problem is
at the Cygwin end, because if I ssh to the Linux box, I can, for
example, connect to and use an (x)emacs instance I launched inside the
XDMCP instance while it was still working.  But the changes to that
editor window are not reflected on what I can see (but not affect) on
the Cygwin viewer.

Any suggestions welcome, wrt configuration, XWin args or debugging
steps I might try, on either side.

And yes, I do know about vnc, and will probably give up and go back to
that, but it has some frustrating niggles of its own...

Thanks,

ht
-- 
       Henry S. Thompson, School of Informatics, University of Edinburgh
                10 Crichton Street, Edinburgh EH8 9AB, SCOTLAND
                           e-mail: ht@inf.ed.ac.uk
                      URL: https://www.ltg.ed.ac.uk/~ht/
 [mail from me _always_ has a .sig like this -- mail without it is forged spam]