diff --git a/src/web/public/constants.js b/src/web/public/constants.js
index 6c8de2e7..ea38016b 100644
--- a/src/web/public/constants.js
+++ b/src/web/public/constants.js
@@ -1023,11 +1023,13 @@ function tabClusterNameSplit(name, label) {
 /**
  * Session-list search: the vertical rail's search box and the sidebar's filter
  * box. Trimmed, case-insensitive substring; a whitespace-only query is no query.
+ * Lower-cased with toLowerCase(), never toLocaleLowerCase(): under a Turkish or
+ * Azeri browser locale "API" lowers to "apı" and a search for "api" would miss it.
  * @param {unknown} query
  * @returns {string} the needle, '' when there is nothing to search for
  */
 function tabSearchNeedle(query) {
-  return typeof query === 'string' ? query.trim().toLocaleLowerCase() : '';
+  return typeof query === 'string' ? query.trim().toLowerCase() : '';
 }
 
 /**
@@ -1056,7 +1058,7 @@ function filterTabSearchRows(rows, query) {
   for (const row of Array.isArray(rows) ? rows : []) {
     const hasSection = row.section !== null && row.section !== undefined;
     if (hasSection && !counts.has(row.section)) counts.set(row.section, 0);
-    const text = typeof row.text === 'string' ? row.text.toLocaleLowerCase() : '';
+    const text = typeof row.text === 'string' ? row.text.toLowerCase() : '';
     const matches = !needle || text.includes(needle);
     if (!matches && row.keep !== true) {
       hidden.add(row.key);
diff --git a/test/tab-rail-search.test.ts b/test/tab-rail-search.test.ts
index 3dfdf12f..a27ee9e7 100644
--- a/test/tab-rail-search.test.ts
+++ b/test/tab-rail-search.test.ts
@@ -16,7 +16,15 @@
  * - Only the grouped rail is a tree: hidden rows and the headers of hidden
  *   groups leave the roving walk and the posinset/setsize count.
  * - The rail matches the NAME (a web tab's title); the sidebar keeps matching
- *   name + working directory.
+ *   name + working directory. Lower-casing never follows the browser locale
+ *   (a Turkish locale lowers "API" to "apı").
+ * - A session row with a tab alert is never hidden, and keeps its group or
+ *   case box on screen (the owner's call on #580).
+ * - Escape in a box that holds text clears the search and closes nothing else:
+ *   the global key handler runs first (capture phase) and must route it.
+ * - A change in what shows redraws the connector lines, which are anchored to
+ *   row positions; an unchanged re-apply does not.
+ * - The sidebar hides a case box its filter emptied, like the rail.
  * - The input is labelled, sits at the top of #tabRail, and every new string
  *   reads in zh-CN.
  *
@@ -126,6 +134,24 @@ describe('CodemanTabSearch (pure)', () => {
     expect(result.counts.get('g')).toBe(0);
     expect(search.filter(null as never, 'x').matchCount).toBe(0);
   });
+
+  it('lower-cases without the browser locale, so a Turkish locale still finds "API"', () => {
+    // A context of its own, whose locale-aware lower-casing behaves like the
+    // Turkish locale (I -> dotless i). The prototypes are this context's alone.
+    const context = vm.createContext({ window: {}, globalThis: {} });
+    vm.runInContext(
+      "String.prototype.toLocaleLowerCase = function () { return String(this).replace(/I/g, '\\u0131').toLowerCase(); };",
+      context
+    );
+    vm.runInContext(read('constants.js'), context, { filename: 'constants.js' });
+    const turkish = (context.window as { CodemanTabSearch: TabSearch }).CodemanTabSearch;
+    expect(vm.runInContext("'API'.toLocaleLowerCase()", context)).toBe('apı');
+
+    expect(turkish.needle('API')).toBe('api');
+    const result = turkish.filter([{ key: 'r', text: 'API Review', section: null }], 'api');
+    expect(result.hidden.size).toBe(0);
+    expect(result.matchCount).toBe(1);
+  });
 });
 
 // ─── The rail, driven through the shipping CodemanApp ────────────────────────