With bug report SF #1118 Mike Robinson reported the tray icon not working on KUbuntu. Quote:
I'm using version 1.8.10 on Kubuntu and the "Integrate with the
messaging menu" doesn't cause the program to appear in the KDE
system tray. When I click to close the application, I can see that it
is still running in the background, and when I try to re-open it, it
immediately pops up the window. I would assume that it is trying to
integrate itself into a non-existent Gnome system tray instead of
playing nice with KDE. I feel having this is essential to KDE users.
And he is right, as reported in other locations too (e.g. Redhat ticket #716358) it turns out that using GtkStatusIcon (the long needed EggTrayIcon replacement introduced in GTK+ 2.10) breaks the tray icon in KDE.
As explained in the Redhat ticket Ubuntu and KDE boldly went ahead an implemented a new XEmbed specification, while GNOME/GTK (and GtkStatusIcon used by Liferea) stayed with the old. While KDE tries to be backwards compatible it still fail with sizing issues. This is known for many different applications with tray icons.
For Liferea we intentionally leave all the complexity to the GtkTrayIcon widget implementation by GTK. There is no other portable way to do it right in GTK+ AFAIK.
And yes, effectively this means you cannot use the tray icon with recent KDE versions. For Ubuntu itself Liferea has libindicate support solving the issue there.