From 1ffd71c5e7e342f83dac2d3ac5df52302f713512 Mon Sep 17 00:00:00 2001
From: Hugo Locurcio <hugo.locurcio@hugo.pro>
Date: Mon, 6 Oct 2025 22:46:45 +0200
Subject: [PATCH] Fix RTD language switcher redirecting to nonexistent
 `/en/4.x/` branch

If we were redirected to `/en/4.x/` as a result of the RTD language selector,
redirect to `/en/stable/` instead, as English does not have a `4.x` branch.

For maintenance reasons, non-English languages have a single `4.x` branch
that is the counterpart of `4.3`, `4.4`, `4.5` and so on.).
---
 404.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/404.rst b/404.rst
index a7507b38c..0cf80c065 100644
--- a/404.rst
+++ b/404.rst
@@ -24,6 +24,15 @@ Page not found
     </p>
 
     <script>
+    // If we were redirected to `/en/4.x/` as a result of the RTD language selector,
+    // redirect to `/en/stable/` instead, as English does not have a `4.x` branch.
+    // (For maintenance reasons, non-English languages have a single `4.x` branch
+    // that is the counterpart of `4.3`, `4.4`, `4.5`, and so on.)
+    if (window.location.pathname.includes('/en/4.x/')) {
+      const newUrl = window.location.pathname.replace('/en/4.x/', '/en/stable/');
+      window.location.replace(newUrl);
+    }
+
     // Check for redirects if on a currently invalid page.
     // This is done in JavaScript, as we exceed Read the Docs' limit for the amount of redirects configurable.
     // When testing this feature on a local web server, replace the URL below with just `/_static/redirects.csv`.