VRR is useful for eliminating screen tearing, besides, it's not like I can disable it for just one application, it's impossible and disabling or enabling it globally requires a reboot which means it's not a viable option at all.
You don't always want the highest refresh rate of your monitor, google VRR and see what it does. Although in case of faf client it should indeed run at 144hz, this is what other windowed applications such as chrome, nvidia control panel, msi afterburner or RTSS do.
I tried the two most reliable fps limiters and while on closer inspection they seem to actually work (if I set the limit to 1 fps I can see how slowly faf client reacts to my mouse movement) the flickering still persists.
Every other windowed application works fine and faf is just a flat, static window with some text and occasional picture or two so I can only conclude there is something wrong with it. No idea what, but surely there must be some difference in comparison to every other windowed app I'm running that causes the problem.
For now I disabled VRR for windowed applications since it seems that the main game I'm using it for somehow still has access to it despite running in borderless window mode. But still some devs may wanna have a look at that.