Here we go: https://en.wikipedia.org/wiki/MediaWiki:Pipe-separator?action=edit

That's a non-breaking space.

Solution: delete that override (after figuring out why it was created and confirming it can be deleted).

And   entity is magically converted to the actual character (so it still works in CSS) by silly code in MessageCache::get(), which allegedly works around some old Firefox bug?

In T210851#4789402, @matmarex wrote:

Solution: delete that override (after figuring out why it was created

The edit summary on the edit that created it seems informative: "stop parserfunctions etc from eating the spaces, and ensure that linewraps occur after the separator, not before".

And   entity is magically converted to the actual character (so it still works in CSS) by silly code in MessageCache::get(), which allegedly works around some old Firefox bug?

The commit summary on rSVN51453: General fix for trailing whitespace in messages, so that we don't have to put… makes me think that we added   and   into a bunch of messages because they were getting screwed up by the edit form, and then the conversion there when fetching the message was added so ->plain() or ->text() would still give spaces rather than HTML entities in contexts where entities would break.

If that's right, then the Firefox bug was that <textarea> was turning literal NBSPs into spaces when the message was edited.

Is this modification present on other projects?

An override for this message exists on these projects:

• https://ar.wikipedia.org/wiki/MediaWiki:Pipe-separator
• https://commons.wikimedia.org/wiki/MediaWiki:Pipe-separator
• https://en.wikipedia.org/wiki/MediaWiki:Pipe-separator
• https://en.wikibooks.org/wiki/MediaWiki:Pipe-separator
• https://es.wikipedia.org/wiki/MediaWiki:Pipe-separator
• https://fa.wikipedia.org/wiki/MediaWiki:Pipe-separator
• https://kk.wikipedia.org/wiki/MediaWiki:Pipe-separator
• https://ksh.wikipedia.org/wiki/MediaWiki:Pipe-separator
• https://no.wikipedia.org/wiki/MediaWiki:Pipe-separator
• https://te.wikipedia.org/wiki/MediaWiki:Pipe-separator
• https://vo.wikipedia.org/wiki/MediaWiki:Pipe-separator
• https://vo.wiktionary.org/wiki/MediaWiki:Pipe-separator
• https://zh-yue.wikipedia.org/wiki/MediaWiki:Pipe-separator

I didn't check closely, but it seems that some of them are identical to the core message, some change the pipe to a different character (e.g. a bullet), but some indeed change one of the spaces to NBSP.

The some local messages replace the first space by &nbsp; (U+00A0) to prevent that the line gets wrapped before the pipe (or similar) symbol. Because U+00A0 is not a whitespace it can not collapse with the whitespace between the two spans.

A change from

.mw-changeslist-links {
	> span:not( :first-child ):before {
		content: '@{msg-pipe-separator}';
	}
}

to

.mw-changeslist-links {
	> span:not( :last-child ):after {
		content: '@{msg-pipe-separator}';
	}
}

solves this problem, because the space after the pipe symbol is not replaces by a no-break space and can collapse with the space between the spans.

https://zh-yue.wikipedia.org/wiki/MediaWiki:Pipe-separator replaced the pipe-separator by the fullwidth form ｜ (U+FF5C) without surrounding spaces. Here the space between the spans can not collapse with a space and leads to a unsymmetrical distance. Maybe a zero-width space between the spans would solve this problem and still prevent screen readers from failures.

Is this still an issue? I can't observe this on any skin anymore. Might have been resolved since this deletion by @MusikAnimal.

Thanks. I will submit some edit requests to the wikis where this still needs a change: https://global-search.toolforge.org/?q=.&regex=1&namespaces=8&title=Pipe-separator