From 6994dc2ce669edf44ffef77092e4dd835a9b7261 Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Wed, 28 Oct 2026 03:46:40 +0100
Subject: [PATCH] fix: send a Lite link without a language to the same page in
 the reader's language instead of a bare 404

---
 lib/__tests__/lite-scripts.test.ts | 24 ++++++++++++++++-
 scripts/lite/lib.mjs               | 42 +++++++++++++++++++++++++-----
 scripts/lite/postbuild.mjs         |  2 +-
 3 files changed, 60 insertions(+), 8 deletions(-)

diff --git a/lib/__tests__/lite-scripts.test.ts b/lib/__tests__/lite-scripts.test.ts
index e879ebb6f..7eb8bf3e2 100644
--- a/lib/__tests__/lite-scripts.test.ts
+++ b/lib/__tests__/lite-scripts.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it } from 'vitest';
+import { describe, expect, it, vi } from 'vitest';
 import { createHash } from 'node:crypto';
 import { mkdtempSync, mkdirSync, writeFileSync, readFileSync, rmSync, existsSync } from 'node:fs';
 import { tmpdir } from 'node:os';
@@ -208,6 +208,28 @@ describe('lite build helpers', () => {
     expect(LITE_PENDING_PATH_KEY).toBe(APP_LITE_PENDING_PATH_KEY);
   });
 
+  it('sends a path without a locale to the same path in the reader locale', () => {
+    const html = buildNotFoundShim({ basePath: '/webmail', locales: ['de', 'en', 'zh-TW'], defaultLocale: 'en' });
+    const script = html.match(/<script>([\s\S]*?)<\/script>/)![1];
+    const run = (pathname: string, languages: string[], stored: string | null = null) => {
+      const replace = vi.fn();
+      const location = { pathname, search: '?a=1', hash: '', replace };
+      const storage = { getItem: () => stored, setItem: () => {} };
+      new Function('location', 'navigator', 'localStorage', 'sessionStorage', script)(
+        location, { languages, language: languages[0] }, storage, storage,
+      );
+      return replace.mock.calls[0]?.[0];
+    };
+    expect(run('/webmail/login', ['de-DE'])).toBe('/webmail/de/login?a=1');
+    expect(run('/webmail/mail/thread/x', ['fr-FR'])).toBe('/webmail/en/mail/thread/x?a=1');
+    expect(run('/webmail/login', ['en'], JSON.stringify({ state: { locale: 'zh-TW' } }))).toBe('/webmail/zh-TW/login?a=1');
+    // With a locale the surface logic applies as before; unknown pages stay a 404.
+    expect(run('/webmail/en/nope', ['de'])).toBeUndefined();
+    expect(run('/webmail/de/mail/thread/x', ['en'])).toBe('/webmail/de/mail/');
+    // Outside the mount nothing happens.
+    expect(run('/other/login', ['de'])).toBeUndefined();
+  });
+
   it('renders host snippets and the README for the built variant', () => {
     expect(buildNginxExample({ basePath: '' })).toContain('(?<surface>mail|calendar|contacts|files|settings)');
     // Root-relative everywhere: a sub-path mount means the files live in <root><basePath>
diff --git a/scripts/lite/lib.mjs b/scripts/lite/lib.mjs
index 08a811f54..6692f6c79 100644
--- a/scripts/lite/lib.mjs
+++ b/scripts/lite/lib.mjs
@@ -297,8 +297,13 @@ export function buildRootRedirect({ basePath = "", locales, defaultLocale = "en"
  * hands the link over (hooks/use-lite-link-segments.ts). Only `<locale>` and
  * `<surface>` from the allowlists ever reach `location.replace`, so a crafted
  * URL cannot turn this into an open redirect.
+ *
+ * A path without a locale (/login, /mail/...) goes to the same path under
+ * the reader's locale, picked as the root index.html picks it; the target
+ * stays on this origin and starts with an allowlisted locale.
  */
-export function buildNotFoundShim({ basePath = "", locales, surfaces = LITE_SURFACES }) {
+export function buildNotFoundShim({ basePath = "", locales, surfaces = LITE_SURFACES, defaultLocale = "en" }) {
+  const fallback = locales.includes(defaultLocale) ? defaultLocale : locales[0];
   return `<!doctype html>
 <html lang="en">
 <head>
@@ -317,6 +322,31 @@ export function buildNotFoundShim({ basePath = "", locales, surfaces = LITE_SURF
     path = path.slice(base.length) || "/";
   }
   var parts = path.split("/").filter(Boolean);
+  if (parts.length > 0 && locales.indexOf(parts[0]) === -1) {
+    var fallback = ${JSON.stringify(fallback)};
+    var pick = function (tag) {
+      if (!tag) return null;
+      tag = String(tag);
+      if (locales.indexOf(tag) !== -1) return tag;
+      var lower = tag.toLowerCase();
+      if (lower.indexOf("zh") === 0) {
+        if (/zh-(tw|hk|mo|hant)/.test(lower) && locales.indexOf("zh-TW") !== -1) return "zh-TW";
+        if (locales.indexOf("zh") !== -1) return "zh";
+      }
+      var short = lower.split("-")[0];
+      for (var i = 0; i < locales.length; i++) if (locales[i].toLowerCase() === short) return locales[i];
+      return null;
+    };
+    var chosen = null;
+    try {
+      var stored = JSON.parse(localStorage.getItem(${JSON.stringify(LOCALE_STORAGE_KEY)}) || "null");
+      chosen = pick(stored && stored.state && stored.state.locale);
+    } catch (e) {}
+    var langs = navigator.languages || [navigator.language];
+    for (var j = 0; !chosen && j < langs.length; j++) chosen = pick(langs[j]);
+    location.replace(base + "/" + (chosen || fallback) + "/" + parts.join("/") + location.search + location.hash);
+    return;
+  }
   if (parts.length < 2 || locales.indexOf(parts[0]) === -1 || surfaces.indexOf(parts[1]) === -1) return;
   var target = base + "/" + parts[0] + "/" + parts[1] + "/";
   // The shell itself is missing (host misconfigured): show the 404 instead of looping.
@@ -326,12 +356,12 @@ export function buildNotFoundShim({ basePath = "", locales, surfaces = LITE_SURF
 })();
 </script>
 <style>
-  body { margin: 0; min-height: 100vh; display: flex; align-items: center; justify-content: center; font-family: system-ui, sans-serif; background: #fff; color: #111; }
-  @media (prefers-color-scheme: dark) { body { background: #0a0a0a; color: #ededed; } }
+  body { margin: 0; min-height: 100vh; display: flex; align-items: center; justify-content: center; font-family: system-ui, sans-serif; background: #fff; color: #0a0a0a; }
+  @media (prefers-color-scheme: dark) { body { background: #0a0a0a; color: #fafafa; } a { background: #fafafa; color: #0a0a0a; } }
   main { text-align: center; padding: 1rem; }
-  h1 { font-size: 2.25rem; margin: 0 0 .5rem; }
-  p { margin: 0 0 1.5rem; opacity: .7; }
-  a { color: inherit; }
+  h1 { font-size: 2.25rem; font-weight: 700; margin: 0 0 .5rem; }
+  p { margin: 0 0 1.5rem; color: #737373; }
+  a { display: inline-block; padding: .5rem 1rem; border-radius: .375rem; background: #0a0a0a; color: #fafafa; font-size: .875rem; font-weight: 500; text-decoration: none; }
 </style>
 </head>
 <body>
diff --git a/scripts/lite/postbuild.mjs b/scripts/lite/postbuild.mjs
index 744b95726..dbe423c25 100644
--- a/scripts/lite/postbuild.mjs
+++ b/scripts/lite/postbuild.mjs
@@ -78,7 +78,7 @@ function staticFiles({ config, basePath, locales, defaultLocale, version, commit
     "index.html": buildRootRedirect({ basePath, locales, defaultLocale }),
     // Replaces Next's default not-found page: only this shim replays deep links
     // on hosts without rewrite rules (see buildNotFoundShim).
-    "404.html": buildNotFoundShim({ basePath, locales }),
+    "404.html": buildNotFoundShim({ basePath, locales, defaultLocale }),
     "_redirects": buildRedirects({ basePath, locales }),
     "_headers": buildHeaders({ basePath, connectSrc }),
     "nginx.conf.example": buildNginxExample({ basePath, connectSrc }),