From f9449f6c8d4f5d174bea4a878632384f2b760c5f Mon Sep 17 00:00:00 2001
From: Craig Jennings <c@cjennings.net>
Date: Sun, 13 Sep 2026 09:46:11 -0500
Subject: fix: match the F display flag case-sensitively

wttrin--effective-display-options checked for an existing F with string-match-p, which honors case-fold-search. That defaults to t, so a lowercase f in wttrin-display-options read as F and the flag was never added. wttr.in flags are case-sensitive (q and Q differ), so I bound case-fold-search to nil inside the helper.

The tests cover a lowercase f under both case-fold settings. They also pin that a non-string options value signals wrong-type-argument rather than building a bad URL.
---
 tests/test-wttrin-hide-follow-line.el | 25 +++++++++++++++++++++++++
 wttrin.el                             |  7 +++++--
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/tests/test-wttrin-hide-follow-line.el b/tests/test-wttrin-hide-follow-line.el
index fcfb906..742a872 100644
--- a/tests/test-wttrin-hide-follow-line.el
+++ b/tests/test-wttrin-hide-follow-line.el
@@ -51,6 +51,31 @@
         (wttrin-hide-follow-line nil))
     (should (equal (wttrin--effective-display-options) ""))))
 
+;; wttr.in flags are case-sensitive (q and Q differ), and string-match-p
+;; honours `case-fold-search', which defaults to t.  A lowercase f must not
+;; read as the F flag, whatever the caller's case-fold setting is.
+(ert-deftest test-wttrin--effective-display-options-boundary-lowercase-f-is-not-f ()
+  "Boundary: a lowercase f in the options does not count as the F flag."
+  (let ((wttrin-display-options "0f")
+        (wttrin-hide-follow-line t)
+        (case-fold-search t))
+    (should (equal (wttrin--effective-display-options) "0fF"))))
+
+(ert-deftest test-wttrin--effective-display-options-boundary-case-fold-nil-same-result ()
+  "Boundary: the result does not depend on the caller's `case-fold-search'."
+  (let ((wttrin-display-options "0f")
+        (wttrin-hide-follow-line t)
+        (case-fold-search nil))
+    (should (equal (wttrin--effective-display-options) "0fF"))))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--effective-display-options-error-non-string-options ()
+  "Error: a non-string options value signals rather than building a bad URL."
+  (let ((wttrin-display-options 'bogus)
+        (wttrin-hide-follow-line t))
+    (should-error (wttrin--effective-display-options) :type 'wrong-type-argument)))
+
 ;;; --------------------------------------------------------------------------
 ;;; wttrin--build-url integration
 ;;; --------------------------------------------------------------------------
diff --git a/wttrin.el b/wttrin.el
index 449fb80..bc67924 100644
--- a/wttrin.el
+++ b/wttrin.el
@@ -532,8 +532,11 @@ ERROR-MSG has no class."
   "Return the wttr.in display flags for the weather-buffer request.
 Starts from `wttrin-display-options' and appends the F flag (\"do not show
 the Follow line\") when `wttrin-hide-follow-line' is non-nil and F is not
-already present, so the two controls compose without duplicating the flag."
-  (let ((opts (or wttrin-display-options "")))
+already present, so the two controls compose without duplicating the flag.
+The match is case-sensitive, because wttr.in treats q and Q as
+different flags."
+  (let ((opts (or wttrin-display-options ""))
+        (case-fold-search nil))
     (if (and wttrin-hide-follow-line (not (string-match-p "F" opts)))
         (concat opts "F")
       opts)))
-- 
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