At some point there was an effect in kwin to fade to a grey (with a
weird vigenette effect) as one logs out.
This was signalled with a fake offscreen window with a special window
ID.
The kwin effect has gone, deleted in Plasma 5.8 with
af80a546bfe258b54468b159bb8cf78f16ca9ebf
(Kwin still contains an effect called logout, but that's merely for the
logout prompt)
I intend to replace this logout effect but as I'm going to start from
scratch we may as well communicate from ksmserver -> kwin with a new API
that works on wayland and and solves some other issues all in one.