I was running into the same problem when I upgraded from Windows 7 to Windows 10. After searching through the settings, I found a solution that appears to resolve this issue.
Seamless Use Windows side-by-side with macOS (no restarting required) on your MacBook ®, MacBook Pro, iMac ®, iMac Pro, Mac mini ® or Mac Pro ®.Share files and folders, copy and paste images and text & drag and drop files between Mac and Windows applications.
To fix this problem:
- Go To - Settings>System>Display
- Turn Off 'Adjust My Screen Brightness Automatically' Setting
Many Mac users are still unaware that you can install Windows 10 on Mac for free from Microsoft perfectly legally. Microsoft doesn’t actually require users to activate Windows 10 with a product key unless you want to customize the look of it. I recently installed the Windows 10 Preview via Bootcamp on both my 2012 iMac and 2012 MBP both with Yosemite. Both installations went fine except for the MBP's screen brightness. It will only go to about 75% brightness even though it says 100% leaving me with a pretty dim screen. Download for Windows (8.8 MB) Download for macOS (11.3 MB) macOS 10.12 Sierra and later. Downloads for older systems: Mac OS X 10.7 Lion - Mac OS X 10.11 El Capitan. Automatic Windows updates require that much space or more. If you have an iMac Pro or Mac Pro with 128GB of memory (RAM) or more, your startup disk needs at least as much free storage space as your Mac has memory.
I hope this helps. If anyone is having problems with Windows 10 bootcamp, I would strongly recommend installing the most up-to-date bootcamp drivers. I lost my F-key functionality after the upgrade, and reinstalling the updated drivers resolved that issue.
The following link explains the process to reinstall updated bootcamp drivers:
Windows Download For Mac
Aug 15, 2015 5:04 PM