Re: Pandoc #4317 side effect

[John MacFarlane <jgm-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org>]

To summarize the issue here, which concerns a change to slide
show formats:

Prior to 2.7, versions of pandoc only put material under the "slide
level" headings on slides.  If you set slide level to 2, then
only material under level-2 headings would appear on slides,
and text under level-1 headings would not appear at all
(though level-1 headings would create "title slides" with
just the section title).

Reveal.js users consistently complained (#4317, #5237) that
this made it hard to structure reveal.js 2D slide shows, in
which it is common to have the "top" slide on each column
contain both a title and some text.

In response, I changed pandoc in 2.7 -- as the changelog says,

> Slide show formats behavior change: content under headers less
> than slide level is no longer ignored, but included in the title
> slide (for HTML slide shows) or in a slide after the title slide
> (for beamer). This change makes possible 2D reveal.js slideshows
> with content in the top slide on each stack (#4317, #5237).

The change to reveal.js has been welcomed by reveal.js users,
but the change to beamer is apparently not welcome by everyone.
The reason I made the change to both formats is because I want
slide shows to work the same no matter what output format you
use. (So you can switch from beamer to reveal.js, for example,
and the slides will look more or less the same.)

I had thought the beamer change was innocuous, since it could
be worked around by people who wanted to include content under
top-level section headings but not in the slides, e.g. by using
a filter to strip out this content.

Unfortunately, Thomas Chevrier's use case can't be handled that
way.  He wants to insert raw tex commands that (a) can't be
stripped out but (b) only work if they're outside the
'frame' environment created for a slide.  I can't think of any
workaround for this.

One possible course of action would be to revert the beamer
change while keeping the reveal.js change. This would potentially
have two bad consequences:

1) slide shows would render differently (in content, not just
form) in reveal.js and beamer.

2) this may break slide shows that were constructed using the
new behavior since 2.7 was released.

A less radical change might be to introduce special Div whose
contents get moved outside the slide itself. Anyway, comments
welcome.