[Development] timing of KDE5 panel in login

[Takao Fujiwara]

I'd like to run a custom panel at bottom right in KDE5 for ibus(propertypanel.vala):
https://github.com/ibus/ibus/tree/master/ui/gtk3

Since KDE5 allocates the owned panel at bottom, I try to get the desktop height using _NET_CURRENT_DESKTOP atom.
My custom panel runs at login time and tries to get the desktop height using _NET_CURRENT_DESKTOP atom but KDE5 panel still is not launched.
How can I get when the KDE5 panel is launched at login time?

Actually I use both the custom panel and KNotifications icon.
When I get the callback of "org.kde.StatusNotifierWatcher" of The KNotifications icon, KDE5 panel still is not allocated.
I also checked the atoms with xlsatoms command but I have no idea to get KDE panel.