According comment #15: Things are properly tested.

Together with comment #24 there is a render bug in kernel 4.8 that shows up at
100% cpu load.
With kernel 4.9 this same bug shows up at 0% / idle cpu load.

With 
af79ad2b1f337a00aa150b993635b10bc68dc842
Merge branch 'sched-core-for-linus' of
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
it changed from "bug shows at 100% load" to "bug shows at 0% load". And the
change is something about the scheduler.
To me this seems likely.

In summery the word "regression" might be wrong.