From cecba5f8d795cf17807a8a321c2e03bb89c5e2cb Mon Sep 17 00:00:00 2001
From: Jason Desrosiers <jdesrosi@gmail.com>
Date: Sat, 3 Oct 2026 15:40:27 -0700
Subject: [PATCH] Only use Fluent bidi isolation for RTL locales

Fluent wraps interpolated values in U+2068/U+2069 isolation marks by
default. These are only needed for right-to-left text and otherwise add
invisible characters to messages. Each translation now declares its
text direction, and isolation is enabled only for "rtl" translations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 src/hyperjump-json-schema.test.js |  6 +++---
 src/localization.js               |  8 ++++----
 src/localization.test.js          | 18 +++++++++++++++++-
 src/translations/index.js         | 11 +++++++++--
 4 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/src/hyperjump-json-schema.test.js b/src/hyperjump-json-schema.test.js
index 7ac8f4c..e7b79e0 100644
--- a/src/hyperjump-json-schema.test.js
+++ b/src/hyperjump-json-schema.test.js
@@ -155,9 +155,9 @@ const isCompatible = (compatibility, versionUnderTest) => {
 
 /** @type (messageId: string, messageParams: MessageParams) => string */
 const getMessage = (function () {
-  const ftl = translations["en-US"];
-  const resource = new FluentResource(ftl);
-  const bundle = new FluentBundle("en-US");
+  const translation = translations["en-US"];
+  const resource = new FluentResource(translation.ftl);
+  const bundle = new FluentBundle("en-US", { useIsolating: translation.direction === "rtl" });
   bundle.addResource(resource);
 
   const disjunction = new Intl.ListFormat("en-US", { type: "disjunction" });
diff --git a/src/localization.js b/src/localization.js
index f6abc3c..1672371 100644
--- a/src/localization.js
+++ b/src/localization.js
@@ -24,12 +24,12 @@ export class Localization {
   /** @type (locale: string) => Localization */
   static forLocale(locale) {
     if (!localizationCache.has(locale)) {
-      const ftl = translations[locale];
-      if (!ftl) {
+      const translation = translations[locale];
+      if (!translation) {
         throw Error(`The ${locale} locale is not supported.`);
       }
-      const resource = new FluentResource(ftl);
-      const bundle = new FluentBundle(locale);
+      const resource = new FluentResource(translation.ftl);
+      const bundle = new FluentBundle(locale, { useIsolating: translation.direction === "rtl" });
       bundle.addResource(resource);
       localizationCache.set(locale, new Localization(locale, bundle));
     }
diff --git a/src/localization.test.js b/src/localization.test.js
index 56bce3d..82900cc 100644
--- a/src/localization.test.js
+++ b/src/localization.test.js
@@ -4,13 +4,19 @@ import { Localization } from "./localization.js";
 
 describe("Localization", () => {
   const fixtureLocale = "fx-TR";
+  const ltrFixtureLocale = "fx-LT";
+  const rtlFixtureLocale = "fx-RT";
 
   beforeAll(() => {
-    translations[fixtureLocale] = `test = unsupported locale`;
+    translations[fixtureLocale] = { ftl: `test = unsupported locale`, direction: "ltr" };
+    translations[ltrFixtureLocale] = { ftl: `type-message = Expected a {$expectedTypes}`, direction: "ltr" };
+    translations[rtlFixtureLocale] = { ftl: `type-message = Expected a {$expectedTypes}`, direction: "rtl" };
   });
 
   afterAll(() => {
     delete translations[fixtureLocale];
+    delete translations[ltrFixtureLocale];
+    delete translations[rtlFixtureLocale];
   });
 
   test("unsupported locale", () => {
@@ -21,4 +27,14 @@ describe("Localization", () => {
     const localization = Localization.forLocale(fixtureLocale);
     expect(() => localization.getBooleanSchemaErrorMessage()).to.throw(Error);
   });
+
+  test("ltr locale doesn't isolate placeables", () => {
+    const localization = Localization.forLocale(ltrFixtureLocale);
+    expect(localization.getTypeErrorMessage(["string"])).to.equal("Expected a string");
+  });
+
+  test("rtl locale isolates placeables", () => {
+    const localization = Localization.forLocale(rtlFixtureLocale);
+    expect(localization.getTypeErrorMessage(["string"])).to.equal("Expected a ⁨string⁩");
+  });
 });
diff --git a/src/translations/index.js b/src/translations/index.js
index ce6443e..889942b 100644
--- a/src/translations/index.js
+++ b/src/translations/index.js
@@ -1,6 +1,13 @@
 import enUS from "./en-US.js";
 
-/** @type Record<string, string> */
+/**
+ * @typedef {{
+ *   ftl: string;
+ *   direction: "ltr" | "rtl";
+ * }} Translation
+ */
+
+/** @type Record<string, Translation> */
 export const translations = {
-  "en-US": enUS
+  "en-US": { ftl: enUS, direction: "ltr" }
 };