Fix TOC #null anchors, incidentally enabling TOC for other markup
What does this MR do and why?
Fixes TOC `#null` anchors break navigation for RST/Me... (#603297 - closed).
Cause
When an .rst file includes a .. contents:: directive,
docutils wraps heading text in <a class="toc-backref"> links:
$ ./gitlab-markup/lib/github/commands/rest2html << RST
.. contents::
Heading A
==========
RST
...
...
<a name="heading-a"></a>
<h2><a class="toc-backref" href="#id1">Heading A</a></h2>
...These links have an href but no id, causing
the frontend TOC to produce a #null anchor.
Approach
Building on Support table of contents button for org-mode f... (!238950 - merged), MarkupHeadingAnchorFilter has been added to
MarkupPipeline to append a.anchor elements to headings.
The frontend TOC targets only a.anchor, filtering out <a> tags that lack an id to prevent the #null anchor.
As a side effect, TOC dropdowns are now available for other markup handled by MarkupPipeline, such as MediaWiki.
Screenshots
Manually verified on GDK-in-a-box:
| Markup | Screenshot |
|---|---|
| RST |  |
RST (with .. contents::) |
 |
| MediaWiki |  |
Sources
Click to expand
RST:
Heading 1
=========
Text.
Heading 2
---------RST (with .. contents::):
.. contents::
Heading A
=========
Text.
Heading B
---------MediaWiki:
= Heading 1 =
== Heading 2 ==
=== Heading 3 ===
==== Heading 4 ====Related
Notes
- I don't work on the frontend regularly — a careful look at this approach would be very helpful.
- No docs updates included — this area isn't well covered by existing documentation. If there are specific docs that should be updated, please flag them.
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