Section titles in running header are conditioned by folios (book class)

[Omikhleia]

Seen "in passing" while fixing #1276 : In the book class, for the "section" command, having the section title in the running header is currently (SILE 0.12) conditioned by not having folios disabled...

  if not SILE.scratch.counters.folio.off then
    SILE.call("right-running-head", {}, function ()
      SILE.call("book:right-running-head-font")
...

IMHO, there shouldn't be such a relation between two decisions that should be independent (having running headers != having page folios). Removing that above-mentioned check without taking any other action would however be a breaking changes: the book class currently disables folios on chapter pages - so currently this behavior is what ensures having neither folios nor section titles on chapter pages (e.g. in the SILE manual)...

FWIW, my own approach to it in my own class implementation is to have noheaders, noheaderthispage etc. in a way similar to the folios, so the decision can be made wholly independent (https://github.com/Omikhleia/omikhleia-sile-packages/blob/main/packages/omiheaders.lua) - one reason being that I do want to be able to get page numbers on chapter pages ;-)

[jodros]

I guess I didn't get it despite looking too simple, like, why isn't it just a matter of removing the if?

[Omikhleia]

Because a book class must be able do disable headers on some pages (typically chapter pages):

Removing that above-mentioned check without taking any other action would however be a breaking changes: the book class currently disables folios on chapter pages

Which is legit in most conventions,

But it also needs to be able to disable page folio on some pages too.
However the functionally should be orthogonal decisions, as they depend on conventions (and book types).