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diff --git a/tests/test-modeline-config-vc-cache-key.el b/tests/test-modeline-config-vc-cache-key.el
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+;;; test-modeline-config-vc-cache-key.el --- Tests for VC modeline cache key -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; The VC modeline cache keys on the file.  A symlink whose target moves to a
+;; different VC tree must invalidate the cache, so the key includes the
+;; resolved `file-truename', not just the symlink path.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'modeline-config)
+
+;;; Normal Cases
+
+(ert-deftest test-modeline-vc-cache-key-includes-truename ()
+  "Normal: the cache key includes the resolved truename of the file."
+  (let ((f (make-temp-file "cj-mlkey-")))
+    (unwind-protect
+        (should (member (file-truename f) (cj/modeline-vc-cache-key f)))
+      (delete-file f))))
+
+;;; Boundary Cases
+
+(ert-deftest test-modeline-vc-cache-key-changes-when-symlink-target-moves ()
+  "Boundary: re-pointing a symlink to a new target changes the cache key.
+The symlink path is identical both times; only its truename differs, so a
+key that ignored the truename would serve a stale VC backend."
+  (let* ((dir (make-temp-file "cj-mlkey-dir-" t))
+         (target-a (expand-file-name "a" dir))
+         (target-b (expand-file-name "b" dir))
+         (link (expand-file-name "link" dir)))
+    (unwind-protect
+        (progn
+          (write-region "" nil target-a)
+          (write-region "" nil target-b)
+          (make-symbolic-link target-a link)
+          (let ((key-a (cj/modeline-vc-cache-key link)))
+            (delete-file link)
+            (make-symbolic-link target-b link)
+            (let ((key-b (cj/modeline-vc-cache-key link)))
+              (should-not (equal key-a key-b)))))
+      (delete-directory dir t))))
+
+(ert-deftest test-modeline-vc-cache-key-stable-for-same-file ()
+  "Boundary: the key is stable across calls for an unchanged file."
+  (let ((f (make-temp-file "cj-mlkey-stable-")))
+    (unwind-protect
+        (should (equal (cj/modeline-vc-cache-key f)
+                       (cj/modeline-vc-cache-key f)))
+      (delete-file f))))
+
+(provide 'test-modeline-config-vc-cache-key)
+;;; test-modeline-config-vc-cache-key.el ends here