When are question titles auto-expanded?

Question

Sometimes, pasting a link into the text auto-expands to the question title, and sometimes it doesn't. For example, to take a question transitively suggested by the related links: Is there an explicit description of a cobordism between $\mathbb{CP}^n$ and $\mathbb{RP}^n\times\mathbb{RP}^n$? . Well, in this case it auto-expanded to the question title, but it seems to me that it doesn't always. One option is simply to enter the question title manually, but (1) that's a pain, especially since MO already knows the question title, and (2) it is invalidated if the title of the linked post is updated (whereas I think that MO updates auto-expanded question titles—but I don't know how to test that without doing something naughty like changing the title of one of my questions just to experiment).

When is the question title auto-expanded? Is there any way to trigger this behaviour when it doesn't happen automatically?

(One situation where the question title isn't auto-expanded appears to be cross-SE links: https://physics.stackexchange.com/questions/552396/fluid-dynamics-for-the-sepiolite ; but I'm sure, although I can't dig up an example, that I've also seen it not being auto-expanded even on links within MO.)

EDIT: @GerhardPaseman's suggestion: How do these primes jump? . OK, it's auto-expanded in a question even with q in place of questions, and no slug.

Answer 1

When is the question title auto-expanded?

Whenever you link in a post (not a comment) to a question, i.e. a link starting with https://mathoverflow.net/q/ or https://mathoverflow.net/questions/, followed by a numeric ID, optionally followed by a slash and additional information. The question must not be deleted, and linking to answers doesn't work.

Links work within a site itself and from a main site to its meta and vice versa. Links from a meta site to Meta Stack Exchange and vice-versa work too.

Is there any way to trigger this behaviour when it doesn't happen automatically?

No, as you already mentioned, you have to copy the title yourself which is rather cumbersome.

whereas I think that MO updates auto-expanded question titles

The update happens automatically, but only if you edit the post with the link itself. At that moment, the Markdown ('source code') of the post is rendered and the system will take the current version of the title.

One final caveat: the question title expansion may work differently in the preview than in the resulting post: Either fix edit preview display to render bare links to more than five sites just like the post renderer, or convert all links ← that is proof that linking to Meta Stack Exchange works.