[revealjs] Setting slide-level does not change the way header in divs are considered as new slides

[cderv]

This issue is a follow up on #6705 after I stumble upon what I think is an undesired for reveal JS slides.

Take this example

---
title: Reveal
---
# Title 1

## Slide 1

Text.

### Sub Slide header

Text.

## Slide 2

Text.

This will correctly give us 1 title slide, then 2 vertical slide, first of which contains a <h3> header

❯ pandoc test.md --to revealjs --slide-level=2
<section>
<section id="title-1" class="title-slide slide level1">
<h1>Title 1</h1>

</section>
<section id="slide-1" class="slide level2">
<h2>Slide 1</h2>
<p>Text.</p>
<h3 id="sub-slide-header">Sub Slide header</h3>
<p>Text.</p>
</section>
<section id="slide-2" class="slide level2">
<h2>Slide 2</h2>
<p>Text.</p>
</section></section>

Now let's add a Fenced div around the h3 part

---
title: Reveal
---
# Title 1

## Slide 1

Text.

::: fragment
### Sub Slide header

Text.
:::

## Slide 2

Text.

We now get the <h3>, embed in a new <section> and not just a <div>

❯ pandoc test.md --to revealjs --slide-level=2
<section>
<section id="title-1" class="title-slide slide level1">
<h1>Title 1</h1>

</section>
<section id="slide-1" class="slide level2">
<h2>Slide 1</h2>
<p>Text.</p>
<section id="sub-slide-header" class="fragment">
<h3>Sub Slide header</h3>
<p>Text.</p>
</section>
</section>
<section id="slide-2" class="slide level2">
<h2>Slide 2</h2>
<p>Text.</p>
</section></section>

From change in pandoc 2.8 and discussion in #6705, I was under the impression that a section slide should be created for header in divs with level below the slide level.

Why this is a problem ?

<section> in revealjs means a new slide. If you generated a presentation using the above content, the slide navigation will be messed up especially if you activated some other revealjs feature that are based sections like history: true

Try above content with fenced div using this command

pandoc test.md --to revealjs --slide-level=2 -Vhistory=true -s -o test.html

If you open presentation an navigate using keyboard, you won't be able to reach the last slide, it will look withing the first ones.

The overview of presentation slides will show an unexpected empty slide

where there is none when not using a fenced div

The trick from @tarleb is still working

::: {.fragment}
<!-- -->
### Text 3

Text.
:::

but I was under the impression from #6705 it should have behave differently if --slide-level=2 was set, and h3 used within Div. I tested with version 2.8 though, and it was already working this way creating sections.

Opening this for discussion, if anyone else agree that this should work differently for revealjs presentation than usual HTML.

[tarleb]

I think it might make sense to create a new function Text.Pandoc.makeSection' that takes an additional Maybe Int and allows to cut-off section generation at a specific level. I'm currently taking a deeper look at section handling, and there might be a better solution.

It seems that the HTML writer always creates sections, but doesn't always keep them around in the final output. I wonder why we are doing it this way?

[jgm]

See also #8097, another manifestation of the complexity we currently have.
I don't remember why we always create sections in the HTML writer, but it may be to get numbering? And then there is #5965.

We could probably special-case this by telling the HTML writer that a Div with class "fragment" should not create a section. Ultimately, though, it would be nice to have a cleaner design.

One natural idea (proposed in #5965) is to add the section divs in T.P.App, prior to calling the writers. But the current design of the HTML writer assumes that we've got the section information in the AST, regardless of --section-divs, so this would require some serious rethinking. There may be other places where this change would also cause problems.

[jgm]

We already special-case for 'column' and 'columns', so I'll just add 'fragment'.