Ghost window animation actor
I encountered another ghost actor that's pretty easy to reproduce, the actor is an StWidget created for window animations in windowManager.js.
Steps to reproduce:
Install Pitivi flatpak (not sure if other versions also trigger the bug) like this:
flatpak install http://flatpak.pitivi.org/pitivi.flatpakref
Open Pitivi, create new project, import any video file and drag it into the timeline
Play the video in the embedded preview player and press the "Detach the viewer" button
Press the fullscreen button of the detached viewer, then press the unfullscreen button again
The fullscreen window will be closed, but the animation StWidget is still visible, above everything else on the monitor. At this point it makes sense to have Control Center opened on a secondary monitor and make the secondary monitor the primary one.