https://bugs.freedesktop.org/show_bug.cgi?id=87625
--- Comment #6 from Luke-Jr luke-jr+freedesktopbugs@utopios.org --- (In reply to Michel Dänzer from comment #5)
Does it still happen with glamor from xserver 1.16 or later?
I have glamor 0.6.0 - is that current? (Gentoo doesn't have anything newer, it seems)
What window/compositing manager are you using?
KWin with compositing disabled.