(In reply to Fireball from comment #15)
> I still experience this with mesa 18.0.0_rc3 and r600 driver.

It is not supposed to be fixed by rc3, since Timothy's patch
041b18cf23a0acf7b0eddf63cd7a2a10192432a1 only got applied after that version
was released.  It also was not yet backported to the 18.0 branch [1], so for
now you need to apply it manually (e.g. by placing it in
/etc/portage/patches/media-libs/mesa-18.0.0_rc3, if you are on Gentoo).

[1]: https://cgit.freedesktop.org/mesa/mesa/log/?h=18.0