Hi, 

I am trying to reproduce the behaviour of the tcolorbox LaTeX package, more specifically breakable boxes (as illustrated here: https://tex.stackexchange.com/a/676607/123770). It works nicely out of the box (pun intended) using `\definetextbackground` and drawing the frame with MetaPost, using counters to keep track of whether the current text is at the top, middle, or bottom of the box. However, I'm having issues when one box ends and another starts on the same page (see more details below). From what I understand, the issue is that counters seem to be updated on a page-wide basis, so each MPgraphic ‘sees’ the values of counters at the end of the current page, which may not be right if another box modifies them. Would anyone know how to resolve this?

Based on two answer on TeX StackExchange (https://tex.stackexchange.com/a/486124/123770 , https://tex.stackexchange.com/questions/377234/context-frame-problems/377261#377261), the following code works well if there is no more than one box per page:

```
\definecounter[pageNumberTop]
\definecounter[pageNumberBottom]

\startuseMPgraphic{mp:axiomframe}
  begingroup;
    for i=1 upto nofmultipars :
      path p;
      p := ( llcorner multipars[i]
             -- lrcorner multipars[i]
             -- urcorner multipars[i]
             -- ulcorner multipars[i]
             -- cycle )
             enlarged (EmWidth,EmWidth) ;
      fill p withcolor boxfillcolor ;
      
      % if the current page is the first one covered by the box...
      if (\pagenumber == \rawcountervalue[pageNumberTop]) :
        % ... if it is also the last one, the box fits entirely on the page; we thus draw the full frame
        if \rawcountervalue[pageNumberTop] == \rawcountervalue[pageNumberBottom] :
          draw p
            withpen pencircle scaled 2pt
            withcolor boxlinecolor ;
       
        % ... otherwise, don't draw the bottom line
        else :
          draw llcorner multipars[i] + (-EmWidth, -EmWidth)
               -- ulcorner multipars[i] + (-EmWidth, EmWidth)
               -- urcorner multipars[i] + (EmWidth, EmWidth)
               -- lrcorner multipars[i] + (EmWidth, -EmWidth)
            withpen pencircle scaled 2pt
            withcolor boxlinecolor ;
        fi ;
        picture pic;
        pic := textext.ulft("\tfd\symbol[info]");
        pic := pic shifted ulcorner multipars[i];
        fill bbox pic withcolor white;
        draw pic;
     
      % if the current page is the last one covered by the box, draw the left, bottom, and right lines
      elseif \pagenumber == \rawcountervalue[pageNumberBottom] :
        draw ulcorner multipars[i] + (-EmWidth, EmWidth)
          -- llcorner multipars[i] + (-EmWidth, -EmWidth)
          -- lrcorner multipars[i] + (EmWidth, -EmWidth)
          -- urcorner multipars[i] + (EmWidth, EmWidth)
          withpen pencircle scaled 2pt
          withcolor boxlinecolor ;

      % if the current page is neither the first one nor the last one, only draw the left and right lines
      else :
        draw llcorner multipars[i] - (EmWidth, EmWidth)
             -- ulcorner multipars[i] + (-EmWidth, EmWidth)
          withpen pencircle scaled 2pt
          withcolor boxlinecolor ;
        draw urcorner multipars[i] + (EmWidth, EmWidth)
             -- lrcorner multipars[i] + (EmWidth, -EmWidth)
          withpen pencircle scaled 2pt
          withcolor boxlinecolor ;
      fi ;
    endfor ;
  setbounds currentpicture to OverlayBox ;
  endgroup;
\stopuseMPgraphic

\definetextbackground
  [theoremFrame]
  [mp=mp:axiomframe,
   location=paragraph,
   backgroundcolor=green,
   framecolor=red,
   before={\setcounter[pageNumberTop][\pagenumber]},
   after={\setcounter[pageNumberBottom][\pagenumber]}]

\definestartstop
  [ColoredBox]
  [before={\blank[2em]\starttheoremFrame},
   after={\stoptheoremFrame\blank[2em]}]
```

For instance, 

```
\startColoredBox
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
\stopColoredBox
```
correctly gives a box split over multiple pages, with the top part of the frame only drawn on the first page and the bottom part only drawn on the last one. 
It also works correctly when adding `\page` between boxes:

```
\starttext
\startColoredBox
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
\stopColoredBox
\page
\startColoredBox
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
\stopColoredBox
```

However,  the following gives an incorrect output:
```
\starttext
\startColoredBox
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
\stopColoredBox
\startColoredBox
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
    \input knuth
\stopColoredBox
```
On page 3, both frames are fully drawn while the first box is a continuation of the one on the previous page and the second one is the start of a box continuing on the next page. I tried several variations without success; my main issue is that (if I understand correctly) the values of counters are the same for the two boxes on page 3, so that at least one of them will be wrong. 

If someone has any ideas on how to resolve this, I'd be really grateful! 
Alternatively, if someone has a solution using Tikz, that would also work - I'd ideally use MetaPost as the tight integration with ConTeXt seems (from my short experience) to make many things easier; but any solution using ConTeXt + a small number of other dependencies is fine.

Cheers,
Florent