F20 on iMac: plymouthd runs, clearing screen unexpectedly

[Nick Urbanik]

Dear Folks,

I have installed Fedora 20 on an iMac alongside Mac OS 10.5.8, and it
works fine, except for the grub2 bugs 903937, 893179, 904668, and most
problematic, the unexpected appearance of plymouthd clearing the
screen, and putting a fedora booting-up image in the middle of the
screen.  This happens with either XFCE or gnome shell, and occurs some
time after logging in and doing some work.  I can get rid of it by
killing plymouthd.

Any ideas on how to investigate the cause, and to troubleshoot this?