I am having trouble figuring out which of the many sub tickets I should post this in, so I am going to start by posting it here.

I am very strongly opposed to adding a space after the colon of page titles. There should be a one to one correspondence between what a page's title actually is and what is displayed to the user.

I think we are in a situation right now where some skins and namespaces see the space and others don't? If so, that is probably even worse because of the inconsistency.

Showing a de-normalized page title sounds like a really bad idea that increases complexity, confusion, and technical debt. I did not think it is a good idea to train users to add this unnecessary space. What if the user starts adding the space in their cirrus searches? In their wikitext when they are writing wikilinks? In their user scripts? If people start adding spaces to their wikilinks in wikitext, then suddenly "insource": searches will start needing two searches instead of one to do things like mass cleanup in AutoWikiBrowser.

The old system is beautiful in its simplicity. There is never a space after the colon. I am having trouble thinking of a reason that it would be worth giving up that simplicity. This seems like it will become a major source of technical debt.

Thanks @Novem_Linguae. These are concerns that were raised at the time and we've kept an eye one.

These changes have been fully deployed to a few dozen major wikis for many months or years (including de, fr, es, ja, it). On major wikis where it is yet to be deployed the beta feature is widely enabled (e.g. 80,000 users on enwiki) and we have yet to see this become an issue.

suddenly "insource": searches will start needing two searches instead of one to do things like mass cleanup in AutoWikiBrowser.

Note that there are already numerous ways you can mutate a link to get a non-canonical form: using localised vs. non-localised namespace ("Talk:" vs "Diskussion:"), first letter capitalisation, underscore <-> space substitution. Anyone relying on a tool to do link replacement is already having to use a regex or a library utility like mw.Title (which is already whitespace-tolerant) to achieve normalisation.

The other situations you mention should all be whitespace-tolerant as well.