Coming from from https://bugs.kde.org/show_bug.cgi?id=354103 and now using Plasma 5.5.2 the tray icons for Thunderbird and caffeine are not visible at all. The shutter tray icon is visible but does not respond to mouse events.
thanks, will look into it. Note there is a way to make thunderbird use SNIs directly (AFAIK a plugin called firetray or something)
Coming from https://bugs.kde.org/show_bug.cgi?id=357147 I uninstalled the package kdebase4-workspace-addons and re-logged in. The shutter icon, the caffeine icon and the Thunderbird icon are visible now and the menu of shutter and Thunderbird are working. I would still keep the bug open for a while because in the recent past the icons stopped working when I spent some time using the desktop.
Another effect: when I close Thunderbird (it uses the Firetray addon by the way), the icon is still visible.
Interestingly enough Thunderbird uses Unity's app indicators but not SNI...
After a fresh restart after closing Thunderbird, also the tray icon disappears which is fine. On the other hand, in the recent past this will stop working again when I use the system a while. Maybe this is related with https://bugs.kde.org/show_bug.cgi?id=356880 and https://bugs.kde.org/show_bug.cgi?id=357419
I installed a new base system (openSUSE 42.1) and now the problem seems to be resolved.