Change 825418 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/core@master] [POC] Typographically correct language-specific namespace separator

https://gerrit.wikimedia.org/r/825418

Example on jawiki:
before

frwiki:
before

The only regression is the selection highlight doesn't include the space. This could be fixed with some JS (swapping out the content on a copy event):

In T315893#8184981, @Esanders wrote:

The only regression is the selection highlight doesn't include the space. This could be fixed with some JS (swapping out the content on a copy event):

Another issue with the CSS approach is that it breaks triple-click to select the whole heading in Chrome (works fine in Firefox).

Test wiki created on Patch demo by ESanders (WMF) using patch(es) linked to this task:
https://patchdemo.wmflabs.org/wikis/be55d10977/w

Change 901688 had a related patch set uploaded (by Esanders; author: Esanders):

[mediawiki/core@master] Alternative CSS approach to make triple click and highlighting work

https://gerrit.wikimedia.org/r/901688

Test wiki created on Patch demo by ESanders (WMF) using patch(es) linked to this task:
https://patchdemo.wmflabs.org/wikis/a8aade8d69/w

Change 901688 abandoned by Esanders:

[mediawiki/core@master] Alternative CSS approach to make triple click and highlighting work

Reason:

squashed into parent

https://gerrit.wikimedia.org/r/901688

DISPLAYTITLE changes should be tested in any implementation of this.

In T315893#8906927, @stjn wrote:

DISPLAYTITLE changes should be tested in any implementation of this.

Per T306440:

The new markup is not added when the title is overridden using the wikitext {{DISPLAYTITLE:…}} or -{T|…}- forms. This may be improved in the future.

The parsing team can better speak to why these two cases are complex to support, but part of it is that is that DISPLAYTITLE allows HTML spanning the namespace and title.

I don't think this is a good idea. Titles are not normally typed with a space between the colon and the title, so I think displaying it with a space between the colon and the title could be confusing.

This also adds unnecessary complexity, both to code, and to the elegant system we currently have where what you see corresponds 1:1 with the proper title.

In fact, I'd recommend rolling back this change that is currently live in the talk namespace on mobile.

Related: T338151: Unexpected space between namespace and talk page title on pages in mobile subdomain

@Novem_Linguae Hypothetically, if the "proper" title was also changed to include a space (e.g. if the canonical URL was https://en.wikipedia.org/wiki/Talk:_Drew_Doughty, if mw.config.get('wgPageName') returned 'Talk:_Drew_Doughty', and so on), would you consider that a better approach?

Change the output of mw.config.get('wgPageName')? That's a massive change that would break all sorts of user scripts and gadgets. I would definitely not be in support of that.

Think of all the if ( mw.config.get('wgPageName') === myString ) type code that would need to be rewritten.

In T315893#8912017, @Novem_Linguae wrote:

I don't think this is a good idea. Titles are not normally typed with a space between the colon and the title, so I think displaying it with a space between the colon and the title could be confusing.

This also adds unnecessary complexity, both to code, and to the elegant system we currently have where what you see corresponds 1:1 with the proper title.

I think with this kind of change, it is mostly a ‘deploy and then see if anyone objects on solid grounds’ approach. If the page title code is still Talk:Cat, there’s nothing that makes a stylistic change to how title gets displayed wrong. And, to be honest, I’ve always found the style we are currently having a bit too technical. It is true that the title should still match the URL, but it does in the proposed change. This is what matters. If that were to change (like, if some browsers copied CSS content), we would need to consider not doing this.

In T315893#8912096, @matmarex wrote:

@Novem_Linguae Hypothetically, if the "proper" title was also changed to include a space (e.g. if the canonical URL was https://en.wikipedia.org/wiki/Talk:_Drew_Doughty, if mw.config.get('wgPageName') returned 'Talk:_Drew_Doughty', and so on), would you consider that a better approach?

This would significantly complicate i18n. frwiki would have to have Discussion_:_Drew_Doughty, jawiki ノート：メインページ and so on. If it could not be expected that all page names (except in mainspace) are prefixed with the namespace name + :, it would be such a disruptive change.

And if it only differed in the top heading, everybody would start imitating it elsewhere (because people hate inconsistencies), increasing the gap between internal and aesthetic names.