ConTeXt: empty line in enumeration

You can use the ad-hoc solution which almost always works in such situations and cancel the whitespace manually using \blank[back,overlay].

\setuplayout[backspace=5cm]
\defineenumeration[example][text=Example,alternative=inmargin]

\starttext
\startexample
  One line before
  \startitemize
    \item Test
  \stopitemize
\stopexample
\startexample
  \blank[back,overlay]
  \startitemize
    \item direct itemize
  \stopitemize
\stopexample
\stoptext

To automate this solution, i.e. to insert \blank[back,overlay] whenever \startENUMERATION is directly followed by \startitemize you can use this. Please don't use this...

\appendvalue{\csname ??constructionstarthandler\endcsname enumeration}{\futurelet\next\checkitemize}
\unexpanded\def\checkitemize{%
    \ifx\next\startitemize
        \blank[back,overlay]%
    \fi
}

First let's try to understand why the extra space is occuring. Roughly speaking, using alternative=inmargin is equivalent to

\noindent\inmargin{ .. title .. }
... content ...

The following example shows the behavior.

\starttext
\noindent
\inmargin{Example 1}
\startitemize
  \item This is how enumeration works.
\stopitemize


\inmargin{Example 2}
\startitemize
  \item This works correctly
\stopitemize
\stoptext

An easy fix it to replace \noindent by \noindentation in strc-con.mkvi or locally:

\unprotect
\startsetups[\??constructionrenderings:\v!margin]
    \let\\=\crlf
    \noindentation
    \inmargin[\c!scope=\v!local]{\flushconstructionheadbox}%
    \useconstructionstyleandcolor\c!style\c!color
    \ignorespaces
\stopsetups
\protect

This fixes the extra new line, but produces a misaglined title:

This can be "fixed" by removing the scope=local:

\unprotect
\startsetups[\??constructionrenderings:\v!margin]
    \let\\=\crlf
    \noindentation
    \inmargin[\c!scope=\v!local]{\flushconstructionheadbox}%
    \useconstructionstyleandcolor\c!style\c!color
    \ignorespaces
\stopsetups
\protect

Here is a complete MWE:

\showboxes

\unprotect
\startsetups[\??constructionrenderings:\v!margin]
    \let\\=\crlf
    \noindentation
    \inmargin{\flushconstructionheadbox}%
    \useconstructionstyleandcolor\c!style\c!color
    \ignorespaces
\stopsetups
\protect

\setuplayout[backspace=5cm]
\defineenumeration[example][text=Example,alternative=inmargin,
titlealign=flushleft]

\starttext
\startexample
  \startitemize
    \item Test
  \stopitemize
\stopexample

\startexample
  Line 1
  \startitemize
    \item Test
  \stopitemize
\stopexample
\stoptext

which gives