What is a NavFrame button and where to find that in the MediaWiki (or MediaWiki extensions) code base?

@Aklapper https://www.mediawiki.org/wiki/Manual:Collapsible_elements#NavFrame; I think the source is this gadget.

Are NavFrames also supposed to be replaced by mw-collapsible?

In T167522#3337206, @Jc86035 wrote:

@Aklapper https://www.mediawiki.org/wiki/Manual:Collapsible_elements#NavFrame

That page says MediaWiki version: ≤ 1.18 so I don't think NavFrame is relevant when it comes to MediaWiki code?
Maybe it is relevant though for some Wikimedia web sites out there though?
Hence clarification (and links) welcome in task descriptions to allow others to understand the scope of this task...

@Aklapper I've elaborated the description. I didn't realize that NavFrames were also deprecated (and I didn't know about the mw-collapsible-content class), so maybe the title should be changed?

I assume the others are kept on enwiki for compatibility purposes.

You'd have to ask enwiki for that. :)

If the latter two types are to be retained

I do not know where to see/find the docs for collapsible. If it's also deprecated then there is nothing to retain.

Currently the only thing it can't do is be aligned to the left, at least without changing site CSS.

Is there a dedicated feature request task about that?

You'd have to ask enwiki for that. :)
I do not know where to see/find the docs for collapsible. If it's also deprecated then there is nothing to retain.

In this edit from October 2016, @Redrose64 says in the summary "we're trying to move from collapsible (which is locally defined) to mw-collapsible (which is part of the MediaWiki software)". So I don't think there's anything to retain from it. All three are still currently in use though.

The information is at the bottom of the manual page, although it's not so much documentation as a script. The manual on Meta hasn't been updated to refer to mw-collapsible yet (they appear to function identically except there's an animation in the newer one), but should be accurate.

Is there a dedicated feature request task about that?

Don't think so. Should one be created?

Another difference (to NavFrame) is that there is no way of centring the header text perfectly without an additional div with a margin containing the text (e.g. in Template:Navbox/sandbox; testcases), which is suboptimal since the width of the button can change depending on the font used. However, I'm not sure if doing this is possible without having to do things like position: relative.

Well, it's getting unclear to me what the scope of this task is... :)
Do you request a consistent appearance of items which requires editing code which has been deprecated for years?
Do you request certain functionality in the non-deprecated code ("can't be aligned to the left")?
Do you request migrating from using deprecated button code to maintained button code in on-wiki content?
Do you request updates to the manual on Meta?

At first I didn't realize the code was deprecated, which is why I suggested making them look consistent. I would therefore request the addition of functionality in the new code so that migration from the old code can be achieved. The Meta manual can be copied from the enwiki version. Filing a bug.