i915 visual corruption

[coypu%sdf.org@localhost]

in the hope of someone else being able to piece together hints:
kernel & normal userland: 8.99.37 amd64

gpu(1): Intel(R) HD Graphics 530, using i915drmkms
gpu(2): nVidia GTX 965M (no driver attached, no magic being done)
legacy boot.

on the ridiculous resolution of 3840x2160.

visual corruption is sometimes apparent:
env LIBGL_ALWAYS_SOFTWARE=1 glmark2 : corrupt video
the corrupt lines seem to be the same spot, unless I move the window.

launching a second glmark2 stops the visual corruption.
killing it immediately restarts it.

this is true even if the second glmark2 instance isn't visible.

Video: https://i.imgur.com/5zOTq6i.mp4


(this appears as mild corruption on firefox, but I'm using glmark2 as a
more better way of reproducing it)