From 159c5a5f6fffac6ee7dad7db1fb3d6f9c97545be Mon Sep 17 00:00:00 2001
From: "YUKI \"Piro\" Hiroshi" <piro.outsider.reflex@gmail.com>
Date: Mon, 6 Jan 2025 15:44:10 +0900
Subject: [PATCH] Apply rightside appearance if the sidebar position detection
 is "auto" and the language is RTL #3689

---
 webextensions/common/common.js   | 17 +++++++++++++++++
 webextensions/sidebar/sidebar.js |  5 ++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/webextensions/common/common.js b/webextensions/common/common.js
index 343b39a34..6d77365cf 100644
--- a/webextensions/common/common.js
+++ b/webextensions/common/common.js
@@ -1200,3 +1200,20 @@ export function isMacOS() {
 export function isWindows() {
   return configs.enableWindowsBehaviors || /^Win/i.test(navigator.platform);
 }
+
+const RTL_LANGUAGES = new Set([
+  'ar',
+  'he',
+  'fa',
+  'ur',
+]);
+
+export function isRTL() {
+  const lang = (
+    navigator.language ||
+    navigator.userLanguage ||
+    //(new Intl.DateTimeFormat()).resolvedOptions().locale ||
+    ''
+  ).split('-')[0];
+  return RTL_LANGUAGES.has(lang);
+}
diff --git a/webextensions/sidebar/sidebar.js b/webextensions/sidebar/sidebar.js
index ea801e0ac..3df68d05c 100644
--- a/webextensions/sidebar/sidebar.js
+++ b/webextensions/sidebar/sidebar.js
@@ -16,6 +16,7 @@ import {
   shouldApplyAnimation,
   loadUserStyleRules,
   isMacOS,
+  isRTL,
   notify,
 } from '/common/common.js';
 import * as ApiTabs from '/common/api-tabs.js';
@@ -91,6 +92,7 @@ const mContextualIdentitiesStyle  = document.querySelector('#contextual-identity
 // allow customiation for platform specific styles with selectors like `:root[data-user-agent*="Windows NT 10"]`
 document.documentElement.dataset.userAgent = navigator.userAgent;
 document.documentElement.classList.toggle('platform-mac', isMacOS());
+document.documentElement.classList.toggle('rtl', isRTL());
 
 {
   const url = new URL(location.href);
@@ -1018,6 +1020,7 @@ async function isSidebarRightSide() {
   const mayBeRight = window.mozInnerScreenX - window.screenX > (window.outerWidth - window.innerWidth) / 2;
   if (configs.sidebarPosition == Constants.kTABBAR_POSITION_AUTO &&
       mayBeRight &&
+      !isRTL() &&
       !configs.sidebarPositionRighsideNotificationShown) {
     if (mTargetWindow != (await browser.windows.getLastFocused({})).id)
       return;
@@ -1053,7 +1056,7 @@ async function isSidebarRightSide() {
     }
   }
   return configs.sidebarPosition == Constants.kTABBAR_POSITION_AUTO ?
-    mayBeRight :
+    (mayBeRight || isRTL()) :
     configs.sidebarPosition == Constants.kTABBAR_POSITION_RIGHT;
 }