Do we actually want this?

It would make for a different UI compared to regular new-topic (where there's no way to immediately set the summary) - there's a case to be made that, if it's so important to allow it to be prefilled, we should probably already expose it there in the first place.

Also, we shouldn't just be (ab)using summary just because we don't allow people to post links in title and summary is a convenient alternative.
The idea of (ab)using summary for those links came up when discussing translatewiki conversion. For their specific usecase, it made sense. And for their specific usecase, they wouldn't need a preload; they could build a form themselves (that would integrate better) and submit everything with Flow API.

I'm not opposed to prefilling the summary per se. But I'd say we need to think about it some more, at least.

I would want to use it as part of a triage system for the workflow I am working on, https://en.wikipedia.org/wiki/Wikipedia:Article_request_workshop, that includes categories according to certain criteria and template parameters being set.

Arguably, some of that stuff should be built directly into Flow, rather than implemented ad-hoc through a template, but allowing pre-filled templates would be a quick solution. The templates could be converted to built-in Flow stuff later.

Other cases:

• proofreading page on frwp, where a link to newbie's sandbox is added to the request (preload the link on the summary is the blocking point to move to Flow)
• any feedback page where we need to know user agent

In T94283#1682017, @Trizek-WMF wrote:

Other cases:

• proofreading page on frwp, where a link to newbie's sandbox is added to the request (preload the link on the summary is the blocking point to move to Flow)

This could be done in the topic title once T59153: Allow links in Flow topic titles is done (I have a patch up).

• any feedback page where we need to know user agent

This can be done in the topic title already.

In T94283#1752977, @Mattflaschen wrote:

In T94283#1682017, @Trizek-WMF wrote:

Other cases:

• proofreading page on frwp, where a link to newbie's sandbox is added to the request (preload the link on the summary is the blocking point to move to Flow)

This could be done in the topic title once T59153: Allow links in Flow topic titles is done (I have a patch up).

Cool :)
I'll see if that is enough for users.

• any feedback page where we need to know user agent

This can be done in the topic title already.

By preloading something?

In T94283#1756608, @Trizek-WMF wrote:

This can be done in the topic title already.

By preloading something?

Yes. User agent is just a plain text string, so it can be preloaded already. See https://www.mediawiki.org/wiki/Flow/Architecture/URLs#Flow_URL_summary .

Since the user agent is dynamic, JS would need to generate that URL, but that would be true if it went into summary too.

In T94283#1756608, @Trizek-WMF wrote:

I'll see if that is enough for users.

So it is not enough. I still have some requests from the Forum des nouveaux to preload contents on the summary field. ATM, they add a template to all new messages on the Forum to tag and categorize all non-answered messages.

Is it complicated to create a feature to preload the summary?

Note they will be good candidates for beta test when we will create filters. :)