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diff --git a/tests/testutil-system-defaults.el b/tests/testutil-system-defaults.el
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+;;; testutil-system-defaults.el --- Shared load harness for system-defaults tests -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;; system-defaults.el has startup side effects (server start, coding-system
+;; setup, recentf, file creation under `user-emacs-directory').  Tests that
+;; assert one of its settings load the module fresh in a sandbox with those
+;; external interactions stubbed, then inspect the variable they own.  This
+;; file holds that harness so each system-defaults test file shares one copy.
+
+;;; Code:
+
+(require 'cl-lib)
+(require 'autorevert)
+(require 'bookmark)
+(require 'server)
+(require 'vc-hooks)
+
+(defvar org-dir nil
+  "Test binding for system-defaults bookmark path setup.")
+
+(defvar user-home-dir nil
+  "Test binding for system-defaults default directory setup.")
+
+(defvar use-package-always-ensure nil
+  "Test binding for use-package package installation policy.")
+
+(defconst test-system-defaults--repo-root
+  (file-name-directory
+   (directory-file-name
+    (file-name-directory (or load-file-name buffer-file-name))))
+  "Repository root, derived from this file's location under tests/.")
+
+(defmacro test-system-defaults--with-load-environment (&rest body)
+  "Run BODY with the side effects of loading system-defaults.el stubbed.
+`user-emacs-directory', `user-home-dir', and `org-dir' are redirected to
+throwaway temp directories so file creation lands in the sandbox."
+  (declare (indent 0))
+  `(let ((user-emacs-directory (file-name-as-directory
+                                (make-temp-file "system-defaults-emacs-" t)))
+         (user-home-dir (file-name-as-directory
+                         (make-temp-file "system-defaults-home-" t)))
+         (org-dir (file-name-as-directory
+                   (make-temp-file "system-defaults-org-" t)))
+         (use-package-always-ensure nil))
+     (cl-letf (((symbol-function 'server-running-p) (lambda (&rest _) t))
+               ((symbol-function 'server-start) #'ignore)
+               ((symbol-function 'set-locale-environment) #'ignore)
+               ((symbol-function 'prefer-coding-system) #'ignore)
+               ((symbol-function 'set-default-coding-systems) #'ignore)
+               ((symbol-function 'set-terminal-coding-system) #'ignore)
+               ((symbol-function 'set-keyboard-coding-system) #'ignore)
+               ((symbol-function 'set-selection-coding-system) #'ignore)
+               ((symbol-function 'set-charset-priority) #'ignore)
+               ((symbol-function 'global-auto-revert-mode) #'ignore)
+               ((symbol-function 'recentf-mode) #'ignore))
+       (unless (fboundp 'use-package)
+         (defmacro use-package (&rest _args) nil))
+       ,@body)))
+
+(defun test-system-defaults--load ()
+  "Load system-defaults.el fresh from the repo for a smoke assertion.
+Call inside `test-system-defaults--with-load-environment'."
+  (load (expand-file-name "modules/system-defaults.el"
+                          test-system-defaults--repo-root)
+        nil t))
+
+(provide 'testutil-system-defaults)
+;;; testutil-system-defaults.el ends here