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I've seen this on 2 macbooks & an HP monitor... the values that come back from display-pixel-* (which uses x-display-pixel-*) seems to be a lie. Specifically, switching from one resolution to another seems to generate a myriad of values... I'm trying to get my lookup table working for OSX's 5 scaling factors, but I've gone through 7-8 by now by going up and down. I've seen similar screwups when I change font size and then ask for frame-width... it often takes a while for the values to change or they're just inconsistent.

What IS consistent is the geometry values that you're using to detect changes. When you call display-monitor-attributes-list it calls a different C function based on os (ns-display-monitor-attributes in my case) and that seems to be more consistent.

Here's what I'm currently using as my hook:

(defun rwd/display-change (_disp)       ; ignore this incoming var
  (interactive)
  (when window-system
    (let* ((attr (frame-monitor-attributes))
           (geom (assoc 'geometry attr))
           (w (nth 3 geom))
           (h (nth 4 geom))
           (disp (format "%sx%s" w h))  ; calculated from actual geometry
           (args (assoc disp rwd/displays))
           (default (assoc "default" rwd/displays)))
      (message "rwd/display-change %s" disp)
      (unless args
        (find-variable 'rwd/displays)
        (insert (format "%S\n" (list disp 'w 'h "Font" 16)))
        (message "Please extend rwd/displays with %S" disp)
        (setq args default))
      (apply 'rwd/display-setup (cdr args))
      (rwd-fix-fullscreen)
      (sleep-for 0.25)
      (rwd-split-smart))))

I'm good for now, but you might want to consider having a geometry-change hook. Happy to send a patch if you want.

I would like to use this script to change font size depending on monitor resolution. The problem is that I am interested in DPI, not in pixles. One can have 2 monitors with same number of pixels but different resolutions (e.g. if one is 15" and other is 17").

Is it possible to detect DPI change and trigger this hook as well?

Related discussion: https://emacs.stackexchange.com/questions/28390/quickly-adjusting-text-to-dpi-changes

Hi,

Thanks for this package - I've been looking for something like this for awhile. It would be good to see on MELPA, so there's my vote!

Currently, the package tracks changes in (frame-monitor-attributes) output, but this provides only partial information in the case of connecting/disconnecting multiple monitors, as it returns changes only for the monitor the main Emacs frame is on. How about using (display-monitor-attributes-list), which would return info for all monitors, instead?