https://bugs.freedesktop.org/show_bug.cgi?id=110897
--- Comment #1 from Richard Thier u9vata@gmail.com --- Created attachment 144510 --> https://bugs.freedesktop.org/attachment.cgi?id=144510&action=edit Second screenshot (visible tile boundary - zbuffer is completely wasted below)
Added second screenshot. You can see the tile here where the blue gear is. Above the line that tile starts and above the line from the next tile-line there is good z-buffer always.
Here I am after some seconds so the whole zbuffer became unusable at this point and that is why things are black. In games that have a skybox I tend to see the skybox instead or far away things instead of black!
Resolution is 1024x768 so you can even measure the size of the tile. I think it is 32x32 or similar.