I flagged this a while ago. The banner replaces the heading element that we use to trigger the sticky header. If we are to fix this, we'd likely want to make a change inside Wikidata-Page-Banner to make sure the heading there is marked up consistently so that Vector's code picks it up.

Change 802825 had a related patch set uploaded (by Bernard Wang; author: Bernard Wang):

[mediawiki/extensions/WikidataPageBanner@master] Add #firstHeading id to banner template

https://gerrit.wikimedia.org/r/802825

Change 802825 merged by jenkins-bot:

[mediawiki/extensions/WikidataPageBanner@master] Add #firstHeading id to banner template

https://gerrit.wikimedia.org/r/802825

I'm still investigating why the sticky header isnt showing. For the first link in the description (https://eu.wikipedia.org/wiki/Txikipedia:Munduko_zazpi_mirariak) I believe its because the sticky header isnt available in that namespace 104. For the second, (https://eu.wikipedia.org/wiki/Atari:Hezkuntza/Lehiaketak/2022/04) I havent found any obvious reason why its not working. I probably have to setup page banners locally to reproduce and debug.

Edit: Ok I figured it out, my previous patch didnt work because the original #firstHeading element was just hidden with CSS, when it really needs to be removed so that there is a unique element to query for for the sticky intersection element

https://eu.wikipedia.org/wiki/Atari:Hezkuntza/Lehiaketak/2022/04 is namespace 100.
https://eu.wikipedia.org/wiki/Txikipedia:Munduko_zazpi_mirariak is namespace 104.

@bwang is correct that the sticky header is not enabled on either of these namespaces:
https://gerrit.wikimedia.org/g/mediawiki/skins/Vector/+/615181154cf668a54220b18e77b994d4d64668b1/resources/skins.vector.es6/stickyHeader.js#467
.

In T306744#8042604, @Jdlrobson wrote:

https://eu.wikipedia.org/wiki/Atari:Hezkuntza/Lehiaketak/2022/04 is namespace 100.
https://eu.wikipedia.org/wiki/Txikipedia:Munduko_zazpi_mirariak is namespace 104.

@bwang is correct that the sticky header is not enabled on either of these namespaces:
https://gerrit.wikimedia.org/g/mediawiki/skins/Vector/+/615181154cf668a54220b18e77b994d4d64668b1/resources/skins.vector.es6/stickyHeader.js#467
.

I think this is okay? I'm not entirely sure what the atari namespace is. For Txikipedia - I suppose it would make sense to have the header appear here for consistency with the main namespace. On the other hand, few of the articles here are long enough to make it particularly useful (by design). @Theklan - any thoughts on this one?

Atari is Portal. Is NS 100, and it's used in all Wikimedia projects. Totally related: T303258, where I explain why portals should be also considered by the design team.

I think that for consistency the sticky header should be shown in any page, regardless of the ns.

Change 807219 had a related patch set uploaded (by Bernard Wang; author: Bernard Wang):

[mediawiki/skins/Vector@master] WIP: Add collapsed TOC to sticky header

https://gerrit.wikimedia.org/r/807219

I tagged the wrong task, pls ignore the patch above

It seems like this issue is still present. To summarize my understanding, there was a patch merged into PageBanner, but it didn't actually fix the issue. From @bwang's comment on that patch:

After some more investigating, there are two issues that need to be fixed for this task to work.

1. Ensure stickyHeader.js can query for a unique "heading" element on pages with page banners. Either by updating the selector used in Vector to be a class, or by adding code to the page banner extension that removes the original h1 element.
2. The template data for the title is empty when page banner is used, still unsure if that's something that can be updated in page banner extension or can be handled in Vector.

Additionally, the stickyHeader namespace logic is handled client-side with this this line

const allowedNamespaceNumbers = [ 0, 2, 4, 10, 12, 14, 100, 828 ];

which doesn't include namespace 104, for pages like: https://eu.wikipedia.org/wiki/Txikipedia:Munduko_zazpi_mirariak.