Graphical artefacts between GDM and GNOME Shell login

[Michael Schwendt]

[Disclaimer: I've looked up the spelling of "artefacts", and it's the British
English spelling I had learned. "artifacts" would be American English.]

After logging in on the GDM screen (not when using auto-login!), before
GNOME Shell is started small coloured rectangles appear at the bottom
of the screen. It looks like garbage. As if something writes into
screen buffer space by accident.

Any idea what that could be?

This may have been like that since F22 Alpha, but I'd switched to
auto-login very soon where it doesn't happen. F21 and older are not
affected.