syneryc doesn't die on shutdown, causes 1m30s delay

Machine:

Standard desktop PC, 1 CPU core, SSD drive.
Xubuntu 15.10 (has been upgraded from previous releases)
Linux pegasus 4.2.0-18-generic #22-Ubuntu SMP Fri Nov 6 18:25:50 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
synergy package: 1.6.2-0ubuntu2

synergyc is started as a Xubuntu "Session and Startup" -> "Application Autostart" entry, the command is "synergyc <ip-address>" - and works normally through the GUI session.

Expected behaviour:

Click shutdown in Xubuntu GUI, machine shuts down and powers off in a timely few seconds.

Actual behaviour:

GUI disappears, machine waits on a text screen for 1m30s before finishing shutdown nornally.

My digging into the problem:

Following http://freedesktop.org/wiki/Software/systemd/Debugging/ , I enabled a debug shell and the systemd debug boot options. On the shell during the shutdown wait, I executed a systemd command showing the running jobs (can't remember what that command was, but) it revealed session-c1.scope was the task running/waiting.

loginctl session-status c1 revealed it was waiting for synergyc to stop.

ps shows synergyc running during the wait, all the way through to the end of the 1m30s expiry.

In normal use with the GUI running, "killall synergyc" would normally successfully kill synergyc. From the debug shell during the wait, it does not - the synergyc process survives.

Reproducible probably over 90% of the time - there has been the odd occasion it has shut down normally.

I realise it's possible this is a bug in something else, but I have to pitch in somewhere and the fact it won't die with a normal kill command during the wait makes me suspect synergyc.