docusaurus: Autogenerated sidebar item from subfolder don't respect `index.md` at the subfolder root

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

It seems that autogenerated items for subfolders containing an index.md file don’t show up as sections.

For instance, on the sample website from the the 5m tutorial, I would have expected this:

const sidebars = {
  tutorialSidebar: [
    "intro",
    {type: 'autogenerated', dirName: 'tutorial-basics'},
    {type: 'autogenerated', dirName: 'tutorial-extras'},
  ],
}

to create two sections in the sidebar because they both contain a _category_.json file but everything is actually flattened at the root. Replacing both _category_.json with an index.md leads to the same results.

Is this expected?

Reproducible demo

CodeSandbox

Steps to reproduce

1. npm init docusaurus@latest my-website classic
2. Update sidebars.js from:

tutorialSidebar: [{type: 'autogenerated', dirName: '.'}],

to

const sidebars = {
  tutorialSidebar: [
    "intro",
    {type: 'autogenerated', dirName: 'tutorial-basics'},
    {type: 'autogenerated', dirName: 'tutorial-extras'},
  ],
}

Expected behavior

I would expect two sections to appear in the sidebar since both subfolders contain a _category_.json.

Actual behavior

The content of the two sections is actually flattened at the root.

NOTE: replacing the _category_.json files with an index.md files leads to the same results.

Your environment

• Docusaurus version used: 2.4.1

Self-service

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