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diff --git a/tests/test-debug-wttrin-mode-line.el b/tests/test-debug-wttrin-mode-line.el
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+;;; test-debug-wttrin-mode-line.el --- Tests for debug-wttrin-mode-line -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025-2026 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for debug-wttrin-mode-line.  This function is a UI-heavy
+;; diagnostic dump, so testing focuses on the two top-level branches:
+;; - When *wttr.in* is missing, no diagnostic buffer is produced.
+;; - When *wttr.in* exists, the diagnostic buffer is created and contains
+;;   the expected section labels.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+(require 'wttrin-debug
+         (expand-file-name "wttrin-debug.el"
+                           (file-name-directory (locate-library "wttrin"))))
+
+;;; Setup and Teardown
+
+(defun test-debug-wttrin-mode-line-setup ()
+  "Setup for debug-wttrin-mode-line tests."
+  (when (get-buffer "*wttr.in*")
+    (kill-buffer "*wttr.in*"))
+  (when (get-buffer "*wttrin-mode-debug*")
+    (kill-buffer "*wttrin-mode-debug*")))
+
+(defun test-debug-wttrin-mode-line-teardown ()
+  "Teardown for debug-wttrin-mode-line tests."
+  (when (get-buffer "*wttr.in*")
+    (kill-buffer "*wttr.in*"))
+  (when (get-buffer "*wttrin-mode-debug*")
+    (kill-buffer "*wttrin-mode-debug*")))
+
+;;; Normal Cases
+
+(ert-deftest test-debug-wttrin-mode-line-normal-no-buffer-skips-diagnostic-buffer ()
+  "When the *wttr.in* buffer is absent, no diagnostic buffer is created."
+  (test-debug-wttrin-mode-line-setup)
+  (unwind-protect
+      (progn
+        (debug-wttrin-mode-line)
+        (should-not (get-buffer "*wttrin-mode-debug*")))
+    (test-debug-wttrin-mode-line-teardown)))
+
+(ert-deftest test-debug-wttrin-mode-line-normal-buffer-exists-creates-diagnostic-buffer ()
+  "When the *wttr.in* buffer exists, the diagnostic buffer is created."
+  (test-debug-wttrin-mode-line-setup)
+  (unwind-protect
+      (progn
+        (with-current-buffer (get-buffer-create "*wttr.in*")
+          (wttrin-mode))
+        (debug-wttrin-mode-line)
+        (should (get-buffer "*wttrin-mode-debug*")))
+    (test-debug-wttrin-mode-line-teardown)))
+
+(ert-deftest test-debug-wttrin-mode-line-normal-diagnostic-buffer-contains-section-labels ()
+  "Diagnostic buffer contains the expected section labels."
+  (test-debug-wttrin-mode-line-setup)
+  (unwind-protect
+      (progn
+        (with-current-buffer (get-buffer-create "*wttr.in*")
+          (wttrin-mode))
+        (debug-wttrin-mode-line)
+        (with-current-buffer "*wttrin-mode-debug*"
+          (let ((contents (buffer-string)))
+            (should (string-match-p "Wttrin Mode-Line Debug Info" contents))
+            (should (string-match-p "Buffer:" contents))
+            (should (string-match-p "Major mode:" contents))
+            (should (string-match-p "mode-name variable:" contents))
+            (should (string-match-p "mode-line-format first 5 elements:" contents)))))
+    (test-debug-wttrin-mode-line-teardown)))
+
+(provide 'test-debug-wttrin-mode-line)
+;;; test-debug-wttrin-mode-line.el ends here