This package watches the current display configuration (name, pixel width and height,
physical dimensions, and work area) and gives your hook a call if it changes.
Intended use case is plugging/unplugging a monitor.

Usage

Require or use-package this.  Make a hook function which takes one argument, a pair of
pixel width and height, like `(1024 . 768)`, then add your hook to
`dispwatch-display-change-hooks`.  You will get called when that changes, eg by removing
or adding a monitor.  Then call `(dispwatch-mode 1)` to get started and `(dispwatch-mode -1)`
to stop.  `(dispwatch-mode)` toggles.

Example

 (defun my-display-changed-hook (disp)
   (message "rejiggering for %s" disp)
   (cond ((equal disp '(3840 . 1080))   ; laptop + ext monitor
          (setq font-size-pt 10))
         ((equal disp '(1920 . 1080))    ; just laptop
          (setq font-size-pt 11))
         (t (message "Unknown display size %sx%s" (car disp) (cdr disp)))))

  (use-package dispwatch
    :config (and
       (add-hook 'dispwatch-display-change-hooks #'my-display-changed-hook)
       (dispwatch-mode 1)))