Markdown Outline/TOC is empty when a .md file has >500 headings (works at 500, fails at 501)

[Ma-XX-oN]

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Bug

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Summary:

The Markdown file header “Table of contents” / “Outline” UI becomes empty when a Markdown document has more than 500 headings. It works with 500 headings, but fails (empty outline) at 501 headings.

I think that the minimum should be at least 2k. Perhaps more, making it easier to navigate a single file library documentation.

My test repo:

md-TOC-Test

Repro files:

• Working case (500 headings): README.md
• Failing case (501 headings): README-501-headings.md

Steps to reproduce:

1. Open the working case file (README.md) in the GitHub web UI (Preview / rendered view).
2. Open the file header “Table of contents” / “Outline” panel.
3. Observe that the outline is populated (it contains entries through “Heading 500”).
4. Open the failing case file (README-501-headings.md) in the GitHub web UI (Preview / rendered view).
5. Open the file header “Table of contents” / “Outline” panel.
6. Observe that the outline is empty (no headings shown).

Expected result:

The outline/TOC should populate for README-501-headings.md the same way it does for README.md.

Actual result:

The outline/TOC panel is empty for README-501-headings.md. (The headings still exist in the document, and the file renders, but the outline list does not populate.)

Observations:

• The failure is based on heading count: 500 works, 501 fails.
• It does not seem to depend on heading levels; just total number of headings.
• Tested in an incognito/private window with the same result.

Related:

• Existing report: “Outline feature on Markdown document no longer populates”
  GitHub Community discussion Outline feature on Markdown document no longer populates #140300

Environment:

• OS: Windows 11
• Browser: Chrome Version 144.0.7559.60 (Official Build) (64-bit)

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[vihaancraftz]

hello may i see yourt markdown

[Ma-XX-oN]

It's in the bug report.

[vihaancraftz]

ok thanks will look into it

[Ma-XX-oN]

For those unaware of this feature, there is a list button at the top right which is an outline button:

Clicking it will list all of the headings in the .md file:

You can then click on the items in the outline pane to jump to the item of interest. You can also type stuff and have it filter items:

When you look at the README-501-headings.md file, that outline pane isn't populated:

[vihaancraftz]

Since this is a limit within GitHub’s internal code, you cannot "fix" the native button yourself. However, you can bypass the problem so that your users can still navigate your document.

Here are the three most effective ways to restore navigation to a Markdown file with more than 500 headings.

1. The "Manual TOC" Fix (Recommended)
   GitHub’s native "Outline" panel is failing, but GitHub still supports standard Markdown anchor links. You can insert a manual Table of Contents at the top of your file.

How to do it: Use a script or a VS Code extension (like "Markdown All in One") to generate a list of links. The format looks like this:

Markdown

My Large Documentation

• Introduction
• Advanced Configuration
• Heading 501

... (rest of the file)