I found this bug report for the status notifier plugin:
https://bugs.launchpad.net/ubuntu/+source/xfce4-statusnotifier-plugin/+bug/1756627
Which had the following note:
"This happens if you have the package "ayatana-indicator-application"
installed. Note that although it appears to be stealing the indicators,
they don't show up in indicator-plugin instead."
based on this I killed the process running the:
"ayatana-indicator-application-service"
with the following results:
- As soon as the above process was killed, the Remmina icon appeared in the
system tray.
- The VLC icon (though it was running) did not appear after killing the
above process, however, exiting and restarting VLC caused its icon to
appear in the system tray as well.
The difference in behavior of the two programs may be due to their using
different frameworks (QT vs GTK).
So it appears (at least for my case) this problem is due to the
status notifier / ayatana-indicator-application problem in the above Ubuntu
bug report.
Shannon C. Dealy | DeaTech Research Inc.
de...@deatech.com | Biotechnology Development Services
Telephone USA: +1 541-929-4089 | USA and the Netherlands
Netherlands: +31 85 208 5570 | www.deatech.com