Sorry to bother you Hans, but the macro is not working as expected...
I can see the cropped text (\underbar + special symbols) and there is an
hyphen symbol in the text only example.
I've added the example code I used for tests with \limitatetext and
\doboundtext.
Peter
% interface=en output=pdftex
% Time-stamp:
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\setupcolors[state=start]
\def\limitatefirstline#1#2#3%
{\hbox\bgroup\strut
\setbox\scratchbox\hbox{#1}%
\ifdim\wd\scratchbox>#2\relax
\setbox\scratchbox\hbox{#3}%
\hsize#2\relax
\advance\hsize-\wd\scratchbox
\setbox\scratchbox\vbox{\forgetall#1}%
\setbox\scratchbox\vsplit\scratchbox to \lineheight
\vbox{\unvbox\scratchbox}%
#3%
\else
#1%
\fi
\egroup}
\def\CropSymbol{\red...}%
\starttext
\setupframed[offset=overlay, height=\struttotal, framecolor=lightgray]
\framed[width=8ex,background=color,backgroundcolor=black]{\white 8ex}
\framed{\limitatefirstline{\underbar{Text dummy}}{8ex}{\CropSymbol}}
\framed{\limitatetext{\underbar{Text dummy}}{8ex}{\CropSymbol}}
\blank
\framed[width=120pt,background=color,backgroundcolor=black]{\white 120 pt}
\framed{\limitatefirstline{\textbar \_\textcircumflex\texttilde \%\$\#\textbraceleft \textbraceright \textpm \copyright \registered \pi \textdegree <>\&}{120pt}{\CropSymbol}}
\blank
\framed[width=110pt,background=color,backgroundcolor=black]{\white 110 pt}
\framed{\limitatefirstline{\textbar \_\textcircumflex\texttilde \%\$\#\textbraceleft \textbraceright \textpm \copyright \registered \pi \textdegree <>\&}{110pt}{\CropSymbol}}
\framed{\limitatefirstline{This text is definitely too long to fit this frame.}{110pt}{\CropSymbol}}
\blank
\framed[width=100pt,background=color,backgroundcolor=black]{\white 100 pt}
\framed{\limitatefirstline{\textbar \_\textcircumflex\texttilde \%\$\#\textbraceleft \textbraceright \textpm \copyright \registered\space \pi \textdegree <>\&}{100pt}{\CropSymbol}}
\stoptext
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