We no longer have magic globals (wgFlowOccupyPages) for this, because we wanted to do it the core way for simplicity.

So unless a whole namespace is Flow (https://ca.wikipedia.org/wiki/Viquip%C3%A8dia:Every_new_talk_page_in_this_namespace_is_Flow!) (note talk page is red and Flow), only pages or revisions (both in the MW core sense of the term) can be Flow. But in order to mark a page Flow it has to exist in the page table.

I don't think we want to create core pages when no Flow entity exists, so we create a description (and thus board workflow) in this case.

The hack was to use a one-character space string (' ') rather than the more correct ('') for the description. That has been fixed.

Suggest declining, or putting a non-blank placeholder description. We want to use the core support here, and I think we're doing so reasonably.

In T117828#1784552, @Mattflaschen wrote:

... But in order to mark a page Flow it has to exist in the page table.

Are you saying this removes redness, so there's no easy fix when Flow occupies an empty talk page like https://www.mediawiki.org/wiki/API_talk:Showing_nearby_wiki_information ?

I don't think we want to create core pages when no Flow entity exists, so we create a description (and thus board workflow) in this case.
...
Suggest declining, or putting a non-blank placeholder description. We want to use the core support here, and I think we're doing so reasonably.

I don't understand this part. The title of this task seems a real bug. I understand it's hard to fix :-)

The other contributing factor to this original report, is possibly the large number of pages on mediawiki that were essentially-empty, except for a single revision which tagged them to use LQT, and hence were converted to Flow.

That's related, and I would love it if there was some way to get the Discussion tab of "empty" former LQT pages to red. I would expect the only way to do that is to delete the discussion page/Flow board just like a regular page; would deleting leave the "knowledge" somewhere that the page should be a Flow board?

In T117828#1784710, @Spage wrote:

would deleting leave the "knowledge" somewhere that the page should be a Flow board?

No, that won't work unless the namespace is using the Flow contentmodel by default.

In T117828#1784710, @Spage wrote:

In T117828#1784552, @Mattflaschen wrote:

... But in order to mark a page Flow it has to exist in the page table.

Are you saying this removes redness, so there's no easy fix when Flow occupies an empty talk page like https://www.mediawiki.org/wiki/API_talk:Showing_nearby_wiki_information ?

Yes, we can not mark a page as "intended to be Flow" if it doesn't exist anywhere.

That's related, and I would love it if there was some way to get the Discussion tab of "empty" former LQT pages to red. I would expect the only way to do that is to delete the discussion page/Flow board just like a regular page; would deleting leave the "knowledge" somewhere that the page should be a Flow board?

Yes, a straight deletion/undeletion of a Flow board should work, since the archive table remembers the content model.

In T117828#1787312, @Quiddity wrote:

In T117828#1784710, @Spage wrote:

would deleting leave the "knowledge" somewhere that the page should be a Flow board?

No, that won't work unless the namespace is using the Flow contentmodel by default.

I saw Quiddity's comment, so I retested it and it works:

1. Go to http://127.0.0.1:8080/wiki/Not_a_Flow_board and verify it is non-existent and not Flow.
2. Enable it using Special:EnableFlow
3. Delete the board.
4. Visit http://127.0.0.1:8080/wiki/Not_a_Flow_board again. It is again non-existent and not Flow.
5. Do a straight undeletion. It is now Flow.