This one is deliberate (technically-implemented in T306440, from the designs in T299814 -- there's discussion of possible search issues starting around T299814#7872316).

They're even looking to expand this to all namespaces, see T313636#8174638 and T315893.

Yeah, it should either be in all of the namespaces or none of them. (I am not opposed to it being in all of them.)

In my opinion, there should be a 1:1 correspondence between how a page's title appears to the user, and how it is typed into the search bar. This breaks that assumption.

The extra space is already supported in the search bar or the address bar. Furthermore we add the spaces with CSS so if you select and copy, you don't get the space.

Is there a reason for this task to be mobile-specific? The space is present on desktop as well.

Yeah, it should either be in all of the namespaces or none of them. (I am not opposed to it being in all of them.)

Personally, I'm all for it being present everywhere. It's already on Talk, which (outside of Mediawiki on some wikis) is the most-popular non-Main namespace anyway.

Interesting. Doesn't have the space:

https://en.wikipedia.org/wiki/Talk:Trading_Places?useskin=vector-2022
https://en.wikipedia.org/wiki/Talk:Trading_Places?useskin=vector
https://en.wikipedia.org/wiki/Talk:Trading_Places?useskin=minerva

Has the space:

https://en.m.wikipedia.org/wiki/Talk:Trading_Places?useskin=vector-2022
https://en.m.wikipedia.org/wiki/Talk:Trading_Places?useskin=vector
https://en.m.wikipedia.org/wiki/Talk:Trading_Places?useskin=minerva

Appears to affect only the mobile subdomain (e.g. en.m.wikipedia.org) rather than any specific skin.

Why so much complexity? Shouldn't a skin be consistent regardless of the subdomain?

Oh, sorry, desktop showing the space is actually going to be depending on whether your user is enrolled in the current DiscussionTools usability improvements a/b test, because it's part of the "page frame" parts. You can see the difference by comparing:

• https://en.wikipedia.org/wiki/Talk:Trading_Places?dtenable=0
• https://en.wikipedia.org/wiki/Talk:Trading_Places?dtenable=1

(On MobileFrontend all the DT features are always enabled.)

Why so much complexity? Shouldn't a skin be consistent regardless of the subdomain?

I agree this should be a generic feature. It was initially built as part of the talk pages visual enhancements, so is rolling out with those, hence the current inconsistencies.