Brightness keeps changing in Windows 10 x64

This was bugging me for years till I found answer in some obscure forum, here it the problem: https://support.microsoft.com/en-us/help/3046889/intel-display-power-saving-technology-settings-incorrect-if-power-source-changed-during-sleep

In case the link drops dead, here is still solution how to fix it

Solution:

• Open your Intel HD

  • The drivers for your integrated Intel HD graphics card. I am aware this is counter intuitive for users who have dedicated graphics card, but the way this whole thing at point of writing is working means that most settings from here are considered 'master' settings.

• Make sure that under battery you have the power saving option disabled.

  • Yes, I know that you might be thinking what kind of hard liquor I was drinking, but trust me, this setting is still in affect when you plug in your laptop into the wall.
  • Make sure to check this from time to time, because I found out that this options get reactivated on updates and some other random triggers.

• If you notice that your screen is still flickering brightens based on background, just unplug the power cable and plug it back it. This will reset some DPST settings and it will stop doing it. This happens only if you use sleep on your laptop. Full shutdown should be fine

The other answers didn't work for me - here's how I fixed it:

1. From your desktop, click the Intel Graphics Settings icon in the bottom right-hand corner.
2. Then from the Intel Graphics Settings dashboard, click on the Power icon.
3. Tick Disable under Display Power Saving Technology.

This is taken from the Reddit source.

I believe it's called 'adaptive brightness' and it's under the power settings advanced options.