The following example demonstrates two problems with alternative=margintext in \setuphead:

1. When used with start/stop sectioning, text following the title may be set on the wrong line.
2. Without regard to the type of sectioning, margintext titles may spill over the left edge of the margin and beyond the page frame.
   

\setuppapersize
    [letter]
    [letter,oversized]
\setuplayout
    [location={middle,middle}]
\showframe
\setuphead
    [chapter]
    [number=no,
     alternative=inmargin]
\setuphead
    [section]
    [
     alternative=margintext,
     insidesection={\blank[-line]},
    ]
\starttext
  \starttitle
      [title={Problem description}]
  \bgroup
  \setupwhitespace[medium]
  \startparagraph
      This demonstrates two problems with
      \type{alternative=margintext} in \tex{setuphead}:
      \startitemize[packed,n]
      \startitem
        When used with start/stop sectioning, text following the
        title may be set on the wrong line.
      \stopitem
      \startitem
        Without regard to the type of sectioning, \type{margintext}
        titles may spill over the left edge of the margin and beyond
        the page frame. (Oddly, \tex{paperwidth} is less than the
        sum of \tex{makeupwidth} and the margin widths and
        distances for both letter and A4 paper.)
      \stopitem
      \stopitemize
  \stopparagraph
  \startparagraph
      With start/stop sectioning, the text following the section
      title may begin one line below the start of the title. That
      can be remedied if there is no whitespace between paragraphs
      with \type{insidesection={\blank[-line]}}, but the remedy
      fails when there is whitespace, and increasing the correction
      has no effect. With traditional sectioning, the text appears
      baseline|-|aligned with the heading, as expected. The
      the correction has no effect in any case with traditional
      sectioning.
  \stopparagraph
  \startparagraph
      It makes no difference in any test how the paragraphs are
      delimited—blank lines, \tex{bpar}/\tex{epar},
      \tex{startparagraph}/\tex{stopparagraph}, or \tex{par}.
  \stopparagraph
  \startparagraph
      Tested with standalone beta 2017.07.17 00:20.
  \stopparagraph
  \egroup
  \page
  \startchapter
      [title={Start/stop sectioning}]
  \startsection[title={Mis\-cel\-la\-neous quo\-ta\-tions}]
  \startparagraph
      \input jojomayer
  \stopparagraph
  \startparagraph
      \input carrol \wordright{No indent no whitespace.}
  \stopparagraph
  \stopsection
  \bgroup
  \setupwhitespace[medium]
  \startsection[title={Miscellaneous quotations}]
  \startparagraph
      \input jojomayer
  \stopparagraph
  \startparagraph
      \input carrol \wordright{No indent medium whitespace.}
  \stopparagraph
  \stopsection
  \egroup
  \bgroup
  \setupindenting[yes,small]
  \startsection[title={Miscellaneous quotations}]
  \startparagraph
      \input jojomayer
  \stopparagraph
  \startparagraph
      \input carrol \wordright{Small indent no whitespace.}
  \stopparagraph
  \stopsection
  \egroup
  \bgroup
  \setupwhitespace[medium]
  \setupindenting[yes,small]
  \startsection[title={Miscellaneous quotations}]
  \startparagraph
      \input jojomayer
  \stopparagraph
  \startparagraph
      \input carrol \wordright{Small indent medium whitespace.}
  \stopparagraph
  \stopsection
  \egroup
  \stopchapter
  \chapter{Traditional sectioning}
  \section{Mis\-cel\-la\-neous quo\-ta\-tions}
      \input jojomayer \par
      \input carrol  \wordright{No indent no whitespace.} \par
      No indent no whitespace. \par
  \bgroup
  \setupwhitespace[medium]
  \section{Miscellaneous quotations}
      \input jojomayer \par
      \input carrol \wordright{No indent medium whitespace.} \par
  \egroup
  \bgroup
  \setupindenting[yes,small]
  \section{Miscellaneous quotations}
      \input jojomayer \par
      \input carrol \wordright{Small indent no whitespace.} \par
  \egroup
  \bgroup
  \setupwhitespace[medium]
  \setupindenting[yes,small]
  \section{Miscellaneous quotations}
      \input jojomayer \par
      \input carrol \wordright{Small indent medium whitespace.} \par
  \egroup
\showlayout
\stoptext