Mac: (x-)display-screens or x-display-list

[David Reitter]

On the Mac, a multi-screen setup is problematic for (Carbon) Emacs.

x-display-screens is hard-coded to return 1, and it seems like (x- 
display-list) also only returns a list of one element.

The code (macterm.c) says "This is display since Mac does not support  
multiple ones."

Therefore, it would make sense to have x-display-screens return the  
true number of screens that are running.
That would also imply that display-pixel-width/height would have to  
return the total dimensions of the usable screen area, and that there  
is a function that gives the coordinates of the left upper corner of  
the total screen area. If I have my main screen on the right, and an  
additional one to the left, x ordinates can range from -1280 to +768.  
(That illustrates that each screen may have different dimensions.)  
Currently, the width/height functions return only the dimensions of  
the 'main' screen.
One deficiency of this method is that the single screens have  
different dimensions, and the usable total screen area is not usually  
rectangular.

Alternative: If we take the term 'display' a little more literally,  
then x-display-screens should indeed return the true number of  
screens (disp_count in mac_initialize_display_info), and display- 
(pixel)-height/width should return the appropriate dimensions for the  
different screens.
However, when positioning a frame, for example, I wouldn't know what  
coordinates I'm allowed to use that way, since I don't know how  
screens map to coordinates. So I'd probably find the first option  
more practical.

It seems like the Windows port has trouble, too.

Maybe I'm misunderstanding the way screens/displays are meant to work  
- please let me know.

Cf:
http://lists.gnu.org/archive/html/help-gnu-emacs/2005-06/msg00227.html
http://lists.gnu.org/archive/html/help-gnu-emacs/2005-06/msg00243.html