This probably will end up as "Windows thing, can't fix" but here goes.

I use DisplayFusion's secondary monitor dim functionality, but I came across a tricky bug that took a lot of swearing, display cable switching, swearing, refresh rate fiddling, and swearing to diagnose.

The situation:
- Nvidia GTX 1080 Ti
- 2560x1440 @ 144 Hz, primary, FreeSync, so G-Sync Compatible (works fine)
- 1920x1080 @ 60 Hz, secondary

The bug:
- When the secondary display is dimmed via DisplayFusion, (a) FreeSync doesn't kick in on the primary, (b) even though it's at 144 Hz, the actual visible refresh rate seems closer to 60.