ConTeXt: Troubles with postponing

Question

My book has:

• full-page images and captions on some pages
• and body text flowing through the other pages. My document is divided into sections, each starting on a new page.

I'm trying to place the full-page images and captions on layers by postponing them just before the section breaks. A typical section has 2 pages of body text and 3 full-page images. I tried using floats, but it was too difficult to find where in the body text to place the floats.

It mostly works well, except that the full-page images sometimes get postponed onto the wrong page. I suspect that the \page in the postponed commands gets it confused. Here's a MWE that shows the problem.

In my MWE, the \startpostponing[+2] should place the words POSTPONED CONTENT on page 3, instead the words are on page 4. When I change the postponing +2 to +1 or +3 it works fine.

Can someone please check if it's like this on their computer?

\setuppapersize[A6, portrait][A3, landscape]
\setuppaper[nx=3,ny=2]
\setuparranging[XY]

\starttext

\startpostponing[+2] 
POSTPONED CONTENT
\page
\stoppostponing

ON PAGE 1
\page
\dorecurse{3}{\input tufte}
\stoptext

Answer 1

A different solution that uses floats instead of \startpostponing to achieve the goals described in the edit:

\setuppapersize[A6, portrait][A3, landscape]
\setuppaper[nx=4,ny=2]
\setuparranging[XY]

\definefloat[endofsection][figure][
    default={somewhere:endofsection},
    minheight=\dimexpr\textheight-2\baselineskip,
    frame=on,
]

\setupfloat[ntop=100]

\setuphead[section][
    before={\page\placenamedfloat[endofsection][endofsection]\page}
]

\starttext

\section{Section One}

\dorecurse{2}{\samplefile{knuth}\par}

\placeendofsection{Caption One}{Content One}

\placeendofsection{Caption Two}{Content Two}

\section{Section Two}

\dorecurse{3}{\samplefile{bryson}\par}

\placeendofsection{Caption Three}{Content Three}

\placeendofsection{Caption Four}{Content Four}

\placeendofsection{Caption Five}{Content Five}

\section{Section Three}

\dorecurse{6}{\samplefile{zapf}\par}

\stoptext

It's a little hacky, so take it as a starting point rather than a finished solution :)

(If using layers is a hard requirement, then you could probably do something with Lua + buffers, but that would be best for another question)

Answer 2

This looks like a bug to me. Setting \c_page_postponed_mode to 0 seems to fix the problem, although this may have other unintended consequences:

\unprotect
\c_page_postponed_mode=0
\protect

\setuppapersize[A6, portrait][A3, landscape]
\setuppaper[nx=3,ny=2]
\setuparranging[XY]

\starttext

\startpostponing[+2]
POSTPONED CONTENT
\page
\stoppostponing

ON PAGE 1
\page
\dorecurse{3}{\input tufte}
\stoptext

I suspect that the issue has something to do with \aftergroup in the definition of \page_otr_construct_and_shipout (page-ini.mkxl:170-175):

     % this is tricky! we need to make sure we are in the output group
     \ifnum\c_page_postponed_mode=\plusone
       \aftergroup\page_postponed_blocks_flush
     \else
       \page_postponed_blocks_flush
     \fi