The SCREEN() and SCREENVISIBLE functions return the position or size of a screen. SCREENVISIBLE() refers to the visible section of the screen, excluding the menu bar and the dock.
The SCREENINDEX() function returns (v10.0+) the actual index of the specified screen, counting from left to right.
SCREEN( index [number OR name], coordinate ) SCREEN( index [number OR name], coordinate, percentage )
The SCREEN index can be referenced by either a number OR a name. The Index number can be 0 for the main screen, and then 1 through n are the screens in orientation order from left to right (and top down for screens at the same position). Index Names are shown below.
The Main screen is set by your macOS System Preferences.
The other screens (monitors) are referenced positionally from the left most screen, which may, or may not, be the Main screen.
|Main or 0||the screen with the primary menu bar (as configured in the Displays preferences).|
|Second||the first from the left which is not the Main screen.|
|Third||the second from the left which is not the Main screen.|
|Internal||the internal screen of the Mac.|
|External||the first from the left which is not an internal screen.|
|Mouse||the screen containing the mouse.|
|Front||the screen containing (the most of) the front window.|
|Back||the first from the left not containing (the most of) the front window.|
|Back2||the second left most screen not containing (the most of) the front window.|
|Positive Calculation||the indexed screen from left to right.|
|Negative Calculation||the indexed screen from right to left.|
|Variable Name||the index value based on the value of the variable (v9.0+).|
Note: Since the Main screen can be in any position:
The SCREEN coordinate can be:
The optional SCREEN percentage parameter specifies a percentage of the width or height to offset by. Offsets are always to the right and down, but negative offsets are allowed.
For example SCREEN(Internal,Left,10%) would be the coordinate of left edge of the internal screen plus 10% of the width of the internal screen. SCREEN(Front,Right,-10%) would be the coordinate of the right edge of the external screen minus 10% of the width of the external screen.
SCREEN sizes and positions are in nominal resolution pixels (so a retina screen will typically have half as many nominal resolution pixels as raw pixels).
Coordinates are in absolute nominal resolution pixels relative to the top left corner of the main screen.