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diff --git a/tests/test-mail-config-mu4e-window.el b/tests/test-mail-config-mu4e-window.el
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+;;; test-mail-config-mu4e-window.el --- mu4e main-view window placement -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; mu4e's main view defaults to a display-buffer-full-frame action that deletes
+;; the window split on launch.  mail-config registers a display-buffer-alist
+;; entry routing the "*mu4e-main*" buffer to the current window instead.  These
+;; verify the entry is registered and that, with it active, displaying the main
+;; buffer under mu4e's own full-frame action no longer collapses a split.
+
+;;; Code:
+
+(require 'ert)
+(require 'mail-config)
+
+(ert-deftest test-mail-config-mu4e-main-entry-registered ()
+  "Normal: the *mu4e-main* alist entry avoids the full-frame action."
+  (let ((entry (assoc "\\`\\*mu4e-main\\*\\'" display-buffer-alist)))
+    (should entry)
+    (should (memq 'display-buffer-same-window (cadr entry)))
+    (should-not (memq 'display-buffer-full-frame (cadr entry)))))
+
+(ert-deftest test-mail-config-mu4e-main-keeps-split ()
+  "Normal: displaying *mu4e-main* under the full-frame action keeps the split.
+The registered alist entry takes precedence over the action passed to
+`display-buffer', so the sibling window survives."
+  (let ((a (get-buffer-create "*mail-win-test-a*"))
+        (main (get-buffer-create "*mu4e-main*")))
+    (unwind-protect
+        (save-window-excursion
+          (delete-other-windows)
+          (set-window-buffer (selected-window) a)
+          (split-window (selected-window) nil t)   ;; side-by-side, batch-friendly
+          (let ((n (count-windows)))
+            (display-buffer main '(display-buffer-reuse-window
+                                   display-buffer-reuse-mode-window
+                                   display-buffer-full-frame))
+            (should (= (count-windows) n))))
+      (ignore-errors (kill-buffer a))
+      (ignore-errors (kill-buffer main)))))
+
+(provide 'test-mail-config-mu4e-window)
+;;; test-mail-config-mu4e-window.el ends here