Sync systemd/D-Bus environment variable updates w/ Emacs daemon

config.el

;; Ensures that any change pushed to the systemd environment (e.g.: via dbus-update-activation-environment) immediately applies to the Emacs daemon process without needing to kill it first
;; This is useful for example if you run Emacs as a systemd user service and it happens to start before your WM, since this normally causes DISPLAY / WAYLAND_DISPLAY to be missing
;; Ensuring the variable is synced will fix issues like GUI apps not launching when invoked by emacs (e.g.: via xdg-open/shell mode)
;; Obviously, this code depends on D-Bus!
(dbus-register-monitor
 :session
 (lambda (&rest args)
   (unless (null args)
     (mapc
      (lambda (var-expression)
        (when (stringp var-expression)
          (let ((splitted (split-string var-expression "=")))
            (setenv (car splitted) (string-join (cdr splitted) "=")))))
      (car args))))
 :path "/org/freedesktop/systemd1"
 :interface "org.freedesktop.systemd1.Manager"
 :member "SetEnvironment")