tl;dr ver. : Device Manager -> Display Adapter -> Intel Graphics -> Uninstall Driver (DO NOT detele from system) -> restart

Noticed display keep changing contrast (not brightness cause I knew I disabled it) ratio while watching YouTube videos. Specifically, wether or not the progress bar show seems to be causing the system to change the contrast of the whole display.

Same thing happens in PhotoShop when differrent sub-window is at the front.


Then reboot. Problem gone.