Change Displays Options preferences on Mac
On your Mac, use the Options pane of Displays System Preferences to set what happens when you press the power button on your display and the brightness buttons on your Mac.
To open this pane, choose Apple menu 
> System Preferences, click Displays, then, in the Displays preferences window on the connected display, click Options.
Note: These preferences are only available on some older Apple Cinema Displays.
Display Power Button | Set what happens when you press the power button on the connected display. | ||||||||||
Brightness Buttons | Set what happens when you press the brightness buttons on the keyboard of your Mac. | ||||||||||
Gather Windows | Moves all open Displays preferences windows to the display where this button is. This button appears only if more than one display is connected to your Mac. | ||||||||||