Suppose, not hypothetically, that you start up some program on your Fedora 42 Cinnamon desktop and it helpfully tells you "<X> requires AppIndicator to run. Please install the AppIndicator plugin for your desktop". You are likely confused, so here are some notes.
'AppIndicator' itself is the name of an application notification protocol, apparently originally from KDE, and some desktop environments may need a (third party) extension to support it, such as the Ubuntu one for GNOME Shell . Unfortunately for me , Cinnamon is not one of those desktops. It theoretically has native support for this, implemented in /usr/libexec/xapps/xapp-sn-watcher, part of Cinnamon's xapps package .
The actual 'AppIndicator' protocol is done over D-Bus, because that's the modern way. Since this started as a KDE thing, the D-Bus name is 'org.kde.StatusNotifierWatcher'. What provides certain D-Bus names is found in /usr/share/dbus-1/services, but not all names are mentioned there and 'org.kde.StatusNotifierWatcher' is one of the missing ones. In this case /etc/xdg/autostart/xapp-sn-watcher.desktop mentions the D-Bus name in its 'Comment=', but that's probably not something you can count on to find what your desktop is (theoretically) using to provide a given D-Bus name. I found xapp-sn-watcher somewhat through luck.
There are probably a number of ways to see what D-Bus names are currently registered and active. The one that I used when looking at this is '
dbus-send --print-reply --dest=org.freedesktop.DBus
/org/freedesktop/DBus org.freedesktop.DBus.ListNames
'. As far as I know, there's no easy way to go from an error message about 'AppIndicator' to knowing that you want 'org.kde.StatusNotifierWatcher'; in my case I read the source of the thing complaining which was helpfully in Python.
(I used the error message to find the relevant section of code , which showed me what it wasn't finding.)
I have no idea how to actually fix the problem, or if there is a program that implements org.kde.StatusNotifierWatcher as a generic, more or less desktop independent program the way that stalonetray does for system tray stuff (or one generation of system tray stuff, I think there have been several iterations of it, cf ).
(Yes, I filed a Fedora bug , but I believe Cinnamon isn't particularly supported by Fedora so I don't expect much. I also built the latest upstream xapps tree and it also appears to fail in the same way. Possibly this means something in the rest of the system isn't working right.)