Regarding kernel versions: I also had noticed instabilities (more errors than usual in dmesg from drm and i915, weird GL/compositing glitches) on my HD4400 Intel with one external monitor and laptop display starting with 5.1.x. I've so far remained on 5.0.3 and have not debugged in depth due to a lack of time, but the issues definitely started after 5.0.3.
