Distortion and tearing effect on internal 4k screen (HP elitebook laptop) with gdm/lightdm

I have a weird distortion and flickering effect happening when embedded videos (inside text content) are running in web browsers or when I scroll in any sort of scrollable content (e.g., webpage, slack, gnome settings). When scrolling, I have a couple of black lines appearing and some tearing / distortion around the lines (appear and disappear quickly). Same effect is visible close to animated / embedded video in webpages. However, when running a video from youtube in full screen, no issues at all.

System is Ubuntu 20.04. I have the same issues with wayland and unity, but not with lxqt (I didn't try KDE or any other yet). Tried a livecd with PopOS, same issue, so it seems it is linked to gdm or lightdm.

I also tried to disable the hardware acceleration in browsers, no change. Tried to had the Option "TearFree" "true" in x11 conf file, no success (screen was just showing purple lines).

Laptop is an Elitebook 1040 g8 with a 4k panel and an intel xe graphics.

$ inxi -G
Graphics:
  Device-1: Intel driver: i915 v: kernel 
  Display: wayland server: X.Org 1.20.13 driver: modesetting 
  unloaded: fbdev,vesa resolution: 2560x1440~60Hz 
  OpenGL: renderer: Mesa Intel Xe Graphics (TGL GT2) v: 4.6 Mesa 21.0.3 

I tried all possible resolutions from 3840 t0 1920 (16/9), same behaviour. Interestingly, I can only set the refresh rate when I use the highest resolution (something like 60, 59 and 40).

Any help appreciated. Thanks

EDIT

Same issue with sddm on kde (and what a hell to remove the whole kubuntu and reconfigure gnome / gdm).

MORE EDIT

LXQT over mutter exhibits the same issues. Apparently, openbox must handle things somewhat differently as it is the only one that does not cause flickering or distortion.

MORE MORE EDIT

I installed 18.04 to try out. No flickering issues but there is a whole lot of other things not working (touchpad not recognised, special fn keys, brightness control, etc). Resolution of screen is stuck in UHD (4k) and can't activated night light.

From that install, I upgraded to 20.04 and surprisingly had no screen tearing (??), but also all the other stuff mentioned above (touchpad, resolution) not working either where from a fresh install in 20.04, screen is acting weird, but all the rest is ok.

Something with display drivers or xorg conf?

For anyone facing the same issue with the same laptop (elitebook x360 1040 UHD intel xe), the combo is to set the screen resolution to 3840X2160, 120Hz and enable the fractional scaling (using 175 personally). No tearing or flickering anymore. Other combinations usually do not work properly, especially the default 200% scaling.

I have no specific setup under xorg.conf.d, use the default gnome session (not Wayland as previously), and the i915 driver (not modsetting as I found elsewhere).

I also force the hardware acceleration in Firefox (not sure if it has any effects, though).