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Re: Plasma5 user sessions

org.freedesktop.Notifications. However on our development release those
messages get displayed as windows instead of the usual bubbles. It seems
that the notification handling is not ready when the autostarts are running.

There is a fix for this in Plasma 5.10 onwards.
Or you can backport the "plasma-workspace/startkde/waitforname"  folder

Make sure any Dbus calls are sent asyncronously, otherwise you'll block your startup.