Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

For a non-category doc page, the front matter sidebar_class_name doesn't work if the file is organized as xxx/index.md[x], while it works as expected when the doc is xxx.md[x].

Related issue: #10864

Reproducible demo

https://codesandbox.io/p/devbox/sleepy-bird-zcj5wl

Steps to reproduce

The first four steps are implemented in the provided CodeSandbox.

1. Create a new docusaurus project.
2. Go to docs/tutorial-basics/create-a-document.md, add sidebar_class_name: this-works and see it works.
3. Add a new file docs/foo/index.md with sidebar_class_name: abcde and see it doesn't apply.
4. Add a new file docs/tutorial-basics/bar/index.md with sidebar_class_name: abcde and see it doesn't apply either.

sidebar_class_name does work on "category" index pages:

5. Go to docs/tutorial-basics/_category_.json, remove the "link" entry. Add a new file docs/tutorial-basics/index.md with sidebar_class_name: this-works-too and see it works.

Expected behavior

sidebar_class_name ("abcde") applied to the sidebar items.

Actual behavior

sidebar_class_name doesn't apply.

Your environment

• Docusaurus version used: 3.8.0/3.8.1

Self-service

• I'd be willing to fix this bug myself.