Symbols panels show markdown headers are nested

[lunarwhite]

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Bug

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Take https://github.com/community/community/blob/main/CODE_OF_CONDUCT.md?plain=1 for an example.

Open Symbols panels, we will see every markdown headers are nested, no matter H1/H2/H3.

## Overview and Purpose

## Pledge

## Standards

### _Best Practices for Maintaining a Strong Community_

### _What is not Allowed_

...

It's weird that symbols didn't keep their original structures.

[muzimuzhi]

It's still the case in Sep 2024. See for example https://github.com/actions/checkout/blob/6d193bf28034eafb982f37bd894289fe649468fc/CHANGELOG.md?plain=1

[muzimuzhi]

Checked on Jun 22, 2025 and the problem persists.

In the Preview tab, there's no plain=1 URL parameter and markdown headers result in an Outline panel. All fine.

In the Code tab, plain=1 parameter is added and markdown headers result in nested symbols in an Symbols panel.