Just wanted to update this thread since I have been dealing with this auto dimming/brightening (adaptive brightness) issue for 3 weeks since getting a Dell Inspiron 7786 laptop. It was driving me absolutely crazy! After researching it myself and then going back and forth with Dell support for almost a month, the solution that finally fixed this annoying issue was to enter the BIOS (restart computer and press F2 at startup), click the Video dropdown menu and disable the "Dynamic Backlight Control". This finally fixed the problem for me and hope it can help others.
This totally worked. Thank you so much!!! It kept changing brightness while I was working with Photoshop and it was really annoying.