\showframe

\starttext

\unexpanded\def\startbufferx{\begingroup}
\unexpanded\def\stopbufferx {\endgroup}

\startbuffer
    first
    \startbufferx
        whatever
    \stopbuffer
x
    last
\stopbuffer
\typebuffer
\startbuffer
    first
    \startbufferx
        whatever
    \stopbufferx
    last
\stopbuffer
\typebuffer
\startbuffer
    first
    \startbufferx
        whatever
    \stopbuffer x
    last
\stopbuffer
\typebuffer
\startbuffer
    first
    \startbufferx
        whatever
    \stopbuffer
    x
    last
\stopbuffer
\typebuffer
\startbuffer
    first
    \startbufferx
        whatever
    \stopbuffer
x
    last
\stopbuffer
\typebuffer

\stoptext
