Disable Automatic Adaptative Brightness Control on Windows 10

The only thing that worked on my Dell Latitude to disable this quite annoying "feature" was going to Settings -> Power and Sleep Settings -> Additional Power Settings and choose the Balanced  plan.
You should also check that in the Advanced Power Settings -> Display -> Enable Adaptative Brightness is disabled.
Choosing a Power Plan kept this option enabled even with Enable Adaptative Brightness disabled.

Update. Sometimes this doesn't work. I found an additional setting to try:
Run regedit and go to:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Intel\Display\igfxcui\profiles\Media\Brighten Movie and Darken Movie.
Then set ProcAmpBrithness to 0 (mine was respectively 30.00 and -30.00).
I'll let you know if it works...