If you have virtual workspaces on macOS and you use the emacs function display-pixel-width it will give the combined with of ALL the monitors you are using. You are better off using (caddr (frame-monitor-attribute 'geometry)), since frame-monitor-attribute gets the specified attribute from the monitor which the current frame is actually on. BTW: The function frame-monitor-attributes returns all the monitor attributes. The function display-monitor-attributes-list returns the attributes for ALL the monitors you are using.
I figured this out when I was debugging a couple of emacs functions to move the current frame left or right on the screen.
(defun tkb-move-frame-left ()
"Move the current frame left by 1/10th the width of the physical montor."
(interactive)
(let* ((left (frame-parameter nil 'left))
(monitor-display-width (caddr (frame-monitor-attribute 'geometry)))
(tenth-width (/ monitor-display-width 10))
(new-left (- left tenth-width)))
(set-frame-parameter nil 'left new-left)))
(defun tkb-move-frame-right ()
"Move the current frame right by 1/10th the width of the physical montor."
(interactive)
(let* ((left (frame-parameter nil 'left))
(monitor-display-width (caddr (frame-monitor-attribute 'geometry)))
(tenth-width (/ monitor-display-width 10))
(new-left (+ left tenth-width)))
(set-frame-parameter nil 'left new-left)))