It's also confusing it that clicking the link doesn't auto-expand the section.

On legacy Vector this is a bit of a confusing process too: https://meta.wikimedia.org/wiki/Talk:Terms_of_use?useskin=vector#Quotations doesn't do anything.

Personally I don't think these should be included in the table of contents at all. In mobile web, we do not collapse headings that are not direct children of the parser output element so perhaps we should do something similar here. There's also an argument to be made that collapsed content shouldn't use headings.

In T330612#8650333, @Jdlrobson wrote:

It's also confusing it that clicking the link doesn't auto-expand the section.

On legacy Vector this is a bit of a confusing process too: https://meta.wikimedia.org/wiki/Talk:Terms_of_use?useskin=vector#Quotations doesn't do anything.

It does for me, on old and new Vector alike: clicking a link to the section, in the TOC or an external link like yours, expands it. If this doesn't work for you, please try debugging it, the code implementing it is here: https://gerrit.wikimedia.org/g/mediawiki/core/+/130ceaf3a07c56b2b1605b51b65de6fb42430a0d/resources/src/jquery/jquery.makeCollapsible.js#165

Personally I don't think these should be included in the table of contents at all. In mobile web, we do not collapse headings that are not direct children of the parser output element so perhaps we should do something similar here. There's also an argument to be made that collapsed content shouldn't use headings.

I think it'd be a mistake to redesign the table of contents completely again in response to this edge case. I think that it should be easy to handle the section being invisible in the code detecting the currently visible section.

I wouldn't consider this bug to be a part of Local-Wiki-Template-And-Gadget-Issues, since the collapsing code is provided by MediaWiki core and intended to be used on pages exactly in this manner. It's only slightly unusual that a heading is collapsed.

Per the web team's quarterly grooming, these tasks are being removed from the team's backlog.

Change #1166217 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/skins/Vector@master] sectionObserver: Handle hidden headings

https://gerrit.wikimedia.org/r/1166217

This issue is also reported on Chinese Wikipedia (link). The affected page is https://zh.wikipedia.org/wiki/%E6%9E%97%E5%A4%95 (林夕). The "填詞作品列表" section (<h2>) contains multiple <h3> that are hidden (display: none) by default. The reporter uploaded a screen recording of the issue at https://streamable.com/g9xcbs.

When scrolling, the active section in TOC oscillates between that <h2> and the correct position. Setting DOM breakpoint on attribute modification reveals the relevant script to be resources/skins.vector.js/sectionObserver.js. Specifically, the logic inside IntersectionObserver chooses a hidden <h3> to be the closestNegativeEntry. That element has its boundingClientRect to be all zero due to its ancestor having display: none.

I am not sure how to narrow this down further or create a simpler test case.

Change #1166217 merged by jenkins-bot:

[mediawiki/skins/Vector@master] sectionObserver: Handle hidden headings

https://gerrit.wikimedia.org/r/1166217