Reproducing Vim channel_fill(...) issue

client.vim

if exists('g:dummy_chan')
    silent! call ch_close(g:dummy_chan)
endif

function! DummyCB(chan, msg)
    echom '>>>> msg: ' . string(a:msg)
endfunction

let g:dummy_chan = ch_open('127.0.0.1:9999',
            \ {'mode': 'json',
            \  'callback': function('DummyCB')})

server.py

import socket
import time

s = socket.socket()
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, True)
s.setsockopt(socket.SOL_TCP, socket.TCP_NODELAY, True)
s.bind(('127.0.0.1', 9999))
s.listen(1)

ss, _ = s.accept()
s.close()

ss.send(b'[0,{"a":1,"b":2}]\r\n')
print('ref msg sent, sleeping')
time.sleep(1)

fragments = [(b'[0,["' + b'x' * 4096 + b'",', 0), (b'"' + b'y' * 1024 + b'"]]\r\n', 0)]
for f, timeout in fragments:
    ss.send(f)
    print('fragment sent, sleeping')
    time.sleep(timeout)

ss.close()