fix: generate IDs for HTML headings without id attribute

[bimakw]

Summary

This PR fixes #27383 where HTML headings like <h1>Title</h1> in markdown files would have empty permalink anchors (showing just # in the URL).

Before:

• <h1>Heading without ID</h1> → permalink shows # (empty)

After:

• <h1>Heading without ID</h1> → permalink shows #heading-without-id

Problem

When users write HTML headings in markdown without an id attribute, the frontend's anchors.ts generates permalink links using heading.id. If the heading has no id, the link becomes just #, which is not useful.

GitHub handles this by auto-generating IDs for HTML headings based on their text content, matching the behavior of Markdown headings.

Solution

Modified processNodeAttrID in modules/markup/html_node.go to:

1. Detect heading tags (h1-h6) without an id attribute
2. Extract the text content from the heading (including nested elements)
3. Generate an ID using the same CleanValue function used for Markdown headings
4. Apply the user-content- prefix as expected by the frontend

This ensures consistent behavior between:

• Markdown headings: ## Title → id="user-content-title"
• HTML headings: <h1>Title</h1> → id="user-content-title"

Testing

Added test cases in modules/markup/html_node_test.go covering:

• HTML headings without id (h1, h2, h3)
• HTML headings with existing id (should preserve and prefix)
• HTML headings with user-content prefix (should not double prefix)
• Headings with special characters
• Headings with nested elements (bold, italic)
• Non-heading elements (should not get id)

All existing tests continue to pass.

Fixes #27383

[silverwind]

One difference from GitHub's implementation is that this id generation is not performed in issue comments, but is performed on Gitea.

I think the reason for this is because in issue comments you have multiple markdown documents on one page, so the generated IDs may not be unique any more. So ideally we should disable this generation mechanism on pages where there can be more than one markdown document.

[bimakw]

good point! baru sadar juga kalo di issue comments bisa duplicate ID karena multiple markdown docs di satu page. mau saya tambahin logic buat skip generation di context issue comments? atau prefer bikin follow-up PR aja?

[wxiaoguang]

See this

[bimakw]

@wxiaoguang got it, jadi perlu skip ID generation untuk issue comments ya. mau saya fix di PR ini atau prefer follow-up PR terpisah?

[wxiaoguang]

Please speak in English

[bimakw]

@wxiaoguang got it, I need to skip ID generation for issue comments. Should I fix it in this PR or would you prefer a separate follow-up PR?

[wxiaoguang]

It should be an easy fix in this PR, see #36233 (comment)

[bimakw]

@wxiaoguang Fixed. Now skipping heading ID generation for issue comments (when markupAllowShortIssuePattern is true). Added tests to verify the behavior.

[wxiaoguang]

Please don't use markupAllowShortIssuePattern, it is an abuse.

You need a clearly defined option to control the new behavior. (no need to add a configurable option, you can hard-code it for wiki and file)

[bimakw]

Thanks for the feedback! I've refactored the implementation to use a dedicated EnableHeadingIDGeneration option instead of abusing markupAllowShortIssuePattern.

Changes:

• Added EnableHeadingIDGeneration field to RenderOptions
• Added WithEnableHeadingIDGeneration() builder method to RenderContext
• Enabled it explicitly in repo_file.go and repo_wiki.go
• Updated tests to use the new explicit option

This makes the option clearly defined and hard-coded for file and wiki pages as requested.