Stability round two: no language links on POST-rendered pages, scoped observers, Vary only where it varies

src/i18n/load.ts

@@ -24,7 +24,7 @@ export const MOUNTED_LOCALES_DIR = join(rootDir, "locales");
 // A catalog file is named for the full locale it holds — sv-SE.ts, never sv.ts — with the script
 // subtag when the language needs one (sr-Latn-RS). Anything else in the folder is a mistake worth
 // stopping for.
-const LOCALE_FILE = /^([a-z]{2,3}(?:-[A-Z][a-z]{3})?-[A-Z]{2})\.ts$/;
+const LOCALE_FILE = /^([a-z]{2,3}(?:-[A-Z][a-z]{3})?-(?:[A-Z]{2}|[0-9]{3}))\.ts$/;
 
 export interface LoadI18nOptions {
   localesDir?: string;
@@ -90,7 +90,7 @@ async function readSet(dir: string, label: string, errors: string[]): Promise<Ma
     if (entry.isDirectory() || entry.name.startsWith(".")) continue;
     const locale = LOCALE_FILE.exec(entry.name)?.[1];
     if (locale === undefined) {
-      errors.push(`${label}: "${entry.name}" is not a locale catalog — name it <language>-<REGION>.ts (e.g. sv-SE.ts)`);
+      errors.push(`${label}: "${entry.name}" is not a locale catalog — name it <language>-<REGION>.ts (sv-SE.ts, es-419.ts, sr-Latn-RS.ts)`);
       continue;
     }
     let mod: { default?: unknown };

src/i18n/view-locals.test.ts

@@ -8,6 +8,7 @@ const request = (overrides: Partial<I18nRequest> = {}): I18nRequest => ({
   locale: "sv-SE",
   localeHref: (href) => href,
   locales: ["en-US", "sv-SE"],
+  method: "GET",
   t: ENGLISH,
   url: new URL("http://localhost/admin/users?q=ada"),
   ...overrides,
@@ -30,3 +31,9 @@ test("dir follows the locale's script", () => {
   assert.equal(i18nLocals(request()).dir, "ltr");
   assert.equal(i18nLocals(request({ locale: "ar-EG" })).dir, "rtl");
 });
+
+test("a page rendered from a POST offers no language links — that URL may have no GET at all", () => {
+  // Following one would dead-end on a 405 (a POST-only route), or silently discard a re-rendered
+  // form's input. The picker renders nothing below two choices, so an empty list hides it.
+  assert.deepEqual(i18nLocals(request({ method: "POST" })).localeSwitch, []);
+});

src/i18n/view-locals.ts

@@ -32,6 +32,7 @@ export interface I18nRequest {
   locale: string;
   localeHref: (href: string) => string;
   locales: string[];
+  method: string; // a page rendered in response to a POST has no linkable URL — see localeSwitch
   t: Translate;
   url: URL;
 }
@@ -50,6 +51,10 @@ export const ENGLISH_LOCALS: I18nLocals = {
 
 export function i18nLocals(ctx: I18nRequest): I18nLocals {
   const here = `${ctx.url.pathname}${ctx.url.search}`;
+  // The picker links to this same page in another language. After a POST that page's URL often has
+  // no GET at all (the admin's recovery-code screen, say), so linking there would dead-end on a 405
+  // — and on a re-rendered form it would silently discard what the user typed. Offer nothing.
+  const linkable = ctx.method === "GET" || ctx.method === "HEAD";
   // ctx.localeHref is a no-op unless the URL asked for a locale, so it is also the honest answer to
   // "did it?" — asking the function that decides keeps the two from drifting apart.
   const carried = ctx.localeHref("/") === "/" ? null : ctx.locale;
@@ -58,7 +63,7 @@ export function i18nLocals(ctx: I18nRequest): I18nLocals {
     locale: ctx.locale,
     localeHref: (href) => ctx.localeHref(href),
     localeParam: carried,
-    localeSwitch: ctx.locales.map((tag) => ({ current: tag === ctx.locale, href: localeHref(here, tag), label: localeLabel(tag), tag })),
+    localeSwitch: linkable ? ctx.locales.map((tag) => ({ current: tag === ctx.locale, href: localeHref(here, tag), label: localeLabel(tag), tag })) : [],
     locales: ctx.locales,
     t: ctx.t,
   };