App Windows switching desktop/panels after Power Safe

Not sure if panel or wm is responsible or whatever...

I have a laptop monitor and an external. Both monitors switch off after some 15 minutes inactivity or so.

Then sometimes some, all or none app windows are switching screens/monitors and in the panel they are also switching or in both panels or in the wrong panel.
Should mean it is not very predictable where the windows are on desktop and in panel, when you came back to use the laptop and monitors become alive. First I have to move around the windows to the screens they have been before.

So at a starting point, can you tell me which xfce part might be responsible, that the windows are not going back where I left them? Is it xfce4-panel or where should I start?