Andreas has tested the patch on 8.5 and it failed.
Here is his message:

Running our stackato.exe in a loop, simply asking for information from
the target, with https (TLS) active the application hangs after about
14-28 iterations, with about 14 iterations per minute, so within 1 to
2 minutes. Symptom of not accepting ^C is the same as before I should
note.

After activating the --debug-http-log it is not hanging itself within
10 minutes anymore.
As that option only activates more output, i.e. introduces delays this
looks as if there is still a race condition present, old or new.

This means that I will still have to use my fix and branch of Tcl 8.5
for the stackato client, instead of head.
Sorry about the bad news.

The rough outline of operations done in the client is:

-1- register tls for https, with http
-2- open a https -async socket to a webserver
-3- read some data data, via readable fileevent
-4- close the socket
-5- format and print data

Note that the iterations I speak of here are always new stackato
processes, with each doing the above. The iteration does NOT happen in
a single stackato process.

The last time I had to investigate the hang happend inside of TLS,
during the open of the socket, i.e. step 2. The TLS transform does
sync read/writes to perform the TLS handshake, without using
fileevents.

I suspect that this is true this time as well.