Comment 29 for bug 439138

the switch of gdm to upstart seemed to be a possible reason for this behaviour change. maybe it's something else though..
anyway, the use of tcflush() in X was added by:

commit 446d9443cea31e493d05c939d0128a8116788468
Author: Adam Jackson <email address hidden>
Date: Wed Nov 5 11:51:06 2008 -0500

    linux: Drain the console fd of data when using evdev for keyboards

    Works around a silly bug in the kernel that causes wakeup storms after
    too many keypresses. Should fix the kernel bug too, but this at least
    keeps the idle wakeup count below 1000/sec.