Well, it's always atomic modesetting that breaks downstream.

Some fixes for 5.1 definitely seem to have improved the situation, as current
drm-next 440e80ce02cde7b810e4eb555768c2d77e7a27c8 shows the severe RedShift
phase stutter again which 5.1.15 does not. Going to retest with 5.3-rc1 or 5.2.