For instance, two fixes I had to add because of this issue:

• in Module:Italiques biologiques
• in Module:Formatage du titre

(These modules generate values to be fed to {{DISPLAYTITLE:}}, italicizing the subject but not the rest of the page title.)

update: I also made a fix on enwiki, in Module:Excerpt slideshow.

Code searches:

• nsText in Scribunto repo
• "function getNsText" in MediaWiki repo

The underscores occur because the nsText property is obtained by just calling Title::getNsText, without further processing. It seems to be the only (to be confirmed) property with such underscores, for example the prefixedText property is already gsubbed to spaces.

This property is not much used, and checking its uses on wikis is quite manageable. Furthermore, at first glance, it seems that changing the property would actually fix things, without ever needing changes on wikis. These underscores were really not expected by module developers.

Change #1053882 had a related patch set uploaded (by Gerrit Patch Uploader; author: Francois Pignon):

[mediawiki/extensions/Scribunto@master] Spaces instead of underscores in nsText property of Title objects

https://gerrit.wikimedia.org/r/1053882

Uploaded patch.

Thankfully, the use of spaces or underscores is well established (and docblock-documented) in the Title class, so it should be very future-proof.

(BTW, I noticed the getNsText() method isn't documented in the manual.)

The correct manual is https://www.mediawiki.org/wiki/Extension:Scribunto/Lua_reference_manual

Feel free to document it there.

No, the manual for the Title.php PHP class, not for the Title library in Scribunto.

edit - I have added it: https://www.mediawiki.org/w/index.php?title=Manual:Title.php&diff=prev&oldid=6635381

Here are 4 more modules on enwiki that are impacted by this issue:

• Module:Commons link – already has a fix because of this issue, see my notes here.
• Module:Category handler/shared – is avoiding title.nsText precisely because of this issue, see this diff. (btw, see also note here)
• Module:Portal – currently affected, but thankfully none of the searched namespaces contains spaces/underscores, see the "badNamespaces" variable here.
• Module:Namespace detect – already has a fix because of this issue, see line 109, that has been added here. The fix may be put more locally, after the nsText on line 103; though, currently it also replaces underscores in the "demospace" template parameter, which is a side-effect we may want to keep.

I reviewed Module:Documentation module on frwiki:

As of now, the issue doesn't bring any bug to this module, but fixing the issue would bring more robustness, and slightly cleaner wikicode output.

I reviewed Module:Database report on enwiki: unaffected, fixing the issue would simply result in slightly cleaner wikicode output.

I've had found some modules on enwiki that needed fixing:

• in Module:Track gauge/autodocument
  • same in the sandbox
  • summary: replace nsText and partialUrl() with fullUrl(query): much simpler, host not hardcoded, does not rely on nsText having underscores, proper URL-escaping of namespace
• and a pending edit request for Module:Category series navigation (module for the significantly used template {{Category series navigation}})

And for reference, some searches I have been using:

• insource:nsText insource:/nsText *\.\. *['"]:/
• insource:nsText insource:/nsText *[~=]+ *['"][A-Za-z]+_/

Another fixed module: https://en.wikipedia.org/w/index.php?title=Module:Automated_taxobox/sandbox&diff=prev&oldid=1244873944
(edit request for the module in production)

Again, the module developer expected that the property contains spaces.

On a related note, in TitleLibraryTests.lua there are tests for subjectNsText, and for talkNsText as well (T180911), but there are no tests for nsText.

The patch will have to be rebased after gerrit 1092899 is merged.