scp and ssh 'cat' stalls at 64k bytes

[Chris Roehrig]

I've upgraded cygwin recently (from a much older version) and am 
encountering a new problem on all my Win10/WIn11 machines.

With openssh and pv installed on cygwin (3.4.7-1):

dd if=/dev/zero bs=1 count=65536 | ssh localhost 'cat > /dev/null'    # 
works
dd if=/dev/zero bs=1 count=65537 | ssh localhost 'cat > /dev/null'    # 
stalls (and anything larger)
dd if=/dev/zero bs=1 count=65537 | ssh localhost 'pv > /dev/null'    # 
replace 'cat' with 'pv' and it works

This happens with or without Windows Firewall enabled, with any input > 
64k, and also remotely from Linux.
It also seems to affect scp to cygwin which stalls if the file is >= 64k