Wiki [[_TOC_]] links are double URL-encoded for non-ASCII headings, causing anchor navigation to fail

Summary

In GitLab CE v18.9.2, TOC links generated by [[_TOC_]] are double URL-encoded for headings containing non-ASCII characters (e.g. Japanese). As a result, clicking a TOC link does not scroll to the corresponding heading.

This is a regression — the same Wiki pages worked correctly before upgrading to 18.9.2.

Steps to reproduce

1. Create a Wiki page with the following content:

[[_TOC_]]

## はじめに

Some content here.

## セットアップ方法

Some content here.

2. Save and open the Wiki page.
3. Click on a TOC link for a Japanese heading.
4. Notice that the page does not scroll to the corresponding heading.
5. Inspect the HTML and compare the TOC link href with the heading id.

What is the current bug behavior?

The TOC <a> tag href is double URL-encoded:

<a href="#%25E3%2581%25AF%25E3%2581%2598%25E3%2582%2581%25E3%2581%25AB">はじめに</a>

The heading id and its anchor href use normal (single) URL encoding:

<h2 id="user-content-はじめに">
  はじめに
  <a href="#%E3%81%AF%E3%81%98%E3%82%81%E3%81%AB" class="anchor"></a>
</h2>

Because the % character in the TOC link is itself percent-encoded (% → %25), the TOC link (#%25E3%25...) does not match the heading anchor (#%E3%81%...), so the browser cannot scroll to the target.

Note: headings with ASCII-only text are not affected.

What is the expected correct behavior?

The TOC link href should use single URL encoding, matching the heading anchor:

<a href="#%E3%81%AF%E3%81%98%E3%82%81%E3%81%AB">はじめに</a>

Environment

• GitLab version: CE v18.9.2 (self-managed)
• Installation method: Omnibus / Docker
• Browser: Chrome 146.0.7680.165 (Official Build) (arm64)