Strange behavior in somewhat complex layout

[Andres Conrado Montoya <andresconrado@gmail.com>]

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Hello list, I'm trying to make something using bits and examples found here
and elsewhere, but I'm hitting a wall. I know there's a flawed logic in my
approach, but I can't figure it out. Any help is appreciated.

Expected behavior of the following MWE:
When starting a new chapter, check if the page is even (left page), if so,
fill it with color, and start the chapter in the next odd page (right
page), removing the header but leaving the footer in that start-chapter
page; and if this is a start-chapter page, use a specific graphic overlay
for the page, but if if not (if its a regular page), use another graphic.

Result:
The header still appears in the start-chapter page, if it comes after the
color filled page. But, if the start-chapter page comes after a regular
(even) page, everything works as expected (no color-filled page, no header
in the start-chapter page).

MWE:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% START MWE

\definecolor[rojo][h=ED553B]
\definehighlight[rojo][color=rojo]

\startMPinclusions
    numeric u ; u := 1.125pt ;
    numeric i ; i := 0.125in ;
    string prevChapterTitle, currentChapterTitle;
    prevChapterTitle := "";
    currentChapterTitle := "";
\stopMPinclusions

\startuseMPgraphic{page:background}
    StartPage;
        currentChapterTitle := "\namedstructurevariable{chapter}{title}";

        if currentChapterTitle <> prevChapterTitle :

            fill (topboundary Page -- cycle)
            enlarged(0,8i)
            shifted (0,-4i)
            withcolor transparent(4, 0.2, \MPcolor{rojo}) ;

            fill (bottomboundary Page -- cycle)
            enlarged(0,1u)
            shifted (0,6i)
            withcolor \MPcolor{rojo} ;

            prevChapterTitle := currentChapterTitle ;

        else:

            fill (topboundary Page -- cycle)
            enlarged(0,0.25u)
            shifted (0,-7i+3u)
            withcolor \MPcolor{rojo} ;

            fill (bottomboundary Page -- cycle)
            enlarged(0,0.25u)
            shifted (0,6i)
            withcolor \MPcolor{rojo} ;

        fi;
    StopPage;
\stopuseMPgraphic

\defineoverlay
  [chapter:background]
  [\useMPgraphic{page:background}]

\setupbackgrounds[page][background=chapter:background]

\definepagebreak[firstpagebreak][yes,header]

\startsetups [chapter:before]
   \doifoddpageelse
     {}
     {\pushbackground[page]
        \setupbackgrounds[page][background=color,backgroundcolor=rojo]
        \page[yes,right]
      \popbackground}
\stopsetups

\setuphead[chapter]
          [width=20em,
          before=\directsetup{chapter:before},
          align={flushleft,nothyphenated,verytolerant},
          style={\tfd\sansbold},
          header=empty,
          page=yes,
          commandbefore={\blank[1.75in]},
          after={\blackrule[color=rojo,height=4pt,width=6em]\blank[1.25in]}]


\setuppagenumbering[alternative=doublesided,location=]

\setupheadertexts
    []
    [{{\small\sans\getmarking[chapter]}}]
    [{{\small\sans\getmarking[chapter]}}]
    []

\setupfootertexts
    []
    [{\rojo{\small\sansbold\pagenumber}}]
    [{\rojo{\small\sansbold\pagenumber}}]
    []

\starttext

\dorecurse{4}{%
  \startchapter[title={A chapter with a really long, long title}]
  \dorecurse{9}{\input{tufte}}
  \stopchapter
  \startchapter[title={A different chapter with a really long, long title}]
  \dorecurse{6}{\input{tufte}}
  \stopchapter}

\stoptext

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% STOP MWE

Thank you for your time.

Andrés Conrado Montoya

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