For example when I turn on "System tray icon" on xfce-power-manager-settings but it show on Notification area by xfce-panel rather than area provided by Xfce4-statusnotifier-plugin. And nm-applet (networkmanager-applet) also have same situation. Maybe I use Archlinux and nm-applet in archlinux doesn't support libappindicator.
Xfpm doesn't use SNI. It has very complex context menu which SNI can't handle. Xfpm provides a panel plugin which is better, so it's not necessary to use systray icon. You are right about Arch. It builds nm-applet without indicator support. There is AUR package network-manager-applet-indicator which includes indicator support so nm-applet will be started as SNI.
(In reply to Viktor Odintsev from comment #1) > Xfpm doesn't use SNI. It has very complex context menu which SNI can't > handle. Xfpm provides a panel plugin which is better, so it's not necessary > to use systray icon. > > You are right about Arch. It builds nm-applet without indicator support. > There is AUR package network-manager-applet-indicator which includes > indicator support so nm-applet will be started as SNI. but I find many gtk applications are not supported included Parole Media Player(v0.9.2), and QT aplications are supported, like vlc (base on gtk) is not supported and vlc-qt(base on qt) is supported.
Qt provides QSystemTrayIcon which supports both SNI and system tray as fallback. GTK doesn't provide SNI support. That means GTK applications don't support SNI unless libappindicator is used (which it the only popular library for SNI in GTK world). This plugin can't display items from system tray if they were designed to be displayed only in system tray.