Add multi-monitor support to EXWM config

config.org

@@ -312,15 +312,49 @@ needs to be set up to install them if they aren't already.
      (use-package yaml-mode)
    #+end_src
 
-* EXWM
-  One must fulfil the meme of doing everything with Emacs...
+* Desktop
+** EXWM
+   One must fulfil the meme of doing everything with Emacs... still
+   got a lot of tweaking to do here before I'm happy.
 
-  #+begin_src emacs-lisp
-    (use-package exwm
-      :config
-      (require 'exwm-config)
-      (exwm-config-default))
-  #+end_src
+   #+begin_src emacs-lisp
+     (use-package exwm
+       :config
+       (require 'exwm-randr)
+       (exwm-randr-enable)
+       (require 'exwm-config)
+       (exwm-config-default))
+   #+end_src
+
+   When I have my laptop connected to a monitor I want the built-in
+   display to turn off, but turn back on when it's disconnected. Turns
+   out this is a total pain.
+
+   To start with we need a function to tell whether a monitor's
+   attached. =exwm-randr= provides [[help:exwm-randr--get-monitors][exwm-randr--get-monitors]], but its
+   result is not -- as I'd expect -- a list of monitors, but instead a
+   rather complicated mess that is (as far as I can tell)
+   undocumented. Rather than trying to figure out what was going on
+   there, I opted for the search-in-shell-command-output route
+
+   #+begin_src emacs-lisp
+     (defun hdmi-connected-p ()
+       (string-match-p "HDMI-2 connected"
+                       (shell-command-to-string "xrandr")))
+   #+end_src
+
+   With that defined, an [[help:exwm-randr-screen-change-hook][exwm-randr-screen-change-hook]] can then be
+   added to turn the built-in display on and off appropriately.
+
+   #+begin_src emacs-lisp
+     (add-hook 'exwm-randr-screen-change-hook
+               (lambda ()
+                 (let ((xrandr-command
+                        (if (hdmi-connected-p)
+                            "xrandr --output eDP-1 --off --output HDMI-2 --auto"
+                            "xrandr --output eDP-1 --auto")))
+                   (start-process-shell-command "xrandr" nil xrandr-command))))
+   #+end_src
 
 * Passwords
   This was a little more work than I expected... =password-store=