Comment 2 for bug 1659331

By reviewing the /etc/lightdm/* files once again I noticed that I had a lightdm-gtk-greeter.conf.dpkg-new file and that lightdm-gtk-greeter.conf was a symlink pointing to somewhere else *).
After renaming the dpkg-new file to lightdm-gtk-greeter.conf the solid blue background was gone. Instead lightdm now showed the greeter background (the "background" as set in the .conf file) for a second or so, then the user's background also for a second or so and then the greeter background again.
I then set the user-background to false in the .conf file. Lightdm now only show the greeter background with a flicker after some two seconds.

By pure chance I found bug #1531224 "lightdm-gtk-greeter flickers once little after lightdm starts", which relates the flicker to the intel graphics. And indeed the boxes I see the symptom on nowadays all have intel graphics. However I am pretty sure that I saw the symptom on my previous desktops -- an i7-2600 with a radion video card and an i7-3930 with an nvidia one.
Anyway, the workaround of changing the acceleration to UXA instead of SNA as mentioned in that bug resolved the flicker for me too. Without the user-background in the .conf file set to false lightdm shows the user's background and with the user-background in the .conf file set to false the greeter background, both without a hitch.

For as far as I am concerned this bug can be closed.
:D

*) For the record: /etc/lightdm/lightdm-gtk-greeter.conf was a symlink to /etc/alternatives/lightdm-gtk-greeter-config-derivative, which in turn is a symlink to a (non-existing) /etc/xdg/xdg-xubuntu/lightdm/lightdm-gtk-greeter.conf file.