daemon startup, new frame configuration, and display-pixel-width

Question

I have a snippet that I use to determine what font size to set based on the resolution of the display, for when my laptop has an external monitor set or not:

(defconst prefs/font-size                  14)
(defconst prefs/font-size/display-laptop   12)
(defconst prefs/font-size/display-external 18)

(defun prefs/use-font-size ()
  (let ((px (display-pixel-width)))
    (cond
     ((eq px 7040) prefs/font-size/display-external)
     ((eq px 3200) prefs/font-size/display-laptop)
     (t prefs/font-size))))

This works great when emacs is started normally; however I'm trying to daemon-ify my configuration. Based on that, I now have a function to set UI-related things on default-frame-alist parameters in a before-make-frame as well as initial-frame-alist at startup:

(defun jl/set-frame-params (frame-alist)
  (add-to-list frame-alist
               `(font .
                      ,(concat (prefs/use-font) "-"
                               (number-to-string (prefs/use-font-size)))))
  ; ... more frame parameters
)


(jl/set-frame-params 'initial-frame-alist)
(add-hook 'before-make-frame-hook ; configure new frames
          #'(lambda ()
              (jl/set-frame-params 'default-frame-alist)))

However, here's the behavior I'm seeing:

• initial daemon load: (display-pixel-width) returns nonsense (10) and the initial emacsclient frame has the font size set incorrectly
• creating a new frame after that via make-frame,(display-pixel-width) returns the correct value, but the font size is still incorrect.
• creating a second new frame with make-frame, the font size is finally correct.

What am I doing wrong here, and how can I get my font size set correctly both for the first frame created on a server, and subsequent frames?

Answer 1

After a little digging and experimentation, I discovered that display-pixel-width takes an optional argument which is the name of the display to test; I wasn't able to get that to work.

However, x-display-pixel-width also takes an optional name, and passing the X Display name (:0) returns the correct value even on daemon startup:

(message "display-pixel-width %d or x-display-pixel-width %d"
  (display-pixel-width)
  (x-display-pixel-width ":0"))

display-pixel-width 10 or x-display-pixel-width 3200

This seems to work fine under XWayland as well.