\marginpar inside figure environment fails

Question

I am working with marginpar to put some comments in the margin of my document. In one case I would like to put a comment on a caption of a figure, a MWE of which looks like this:

\begin{document}
 \begin{figure}
 \caption{captiontext  \marginpar{marginText}}
 \end{figure}
\end{document}

Unfortunately, this fails saying: Argument of \@caption has an extra }

Why is this happening? And how can I fix it/work around it?

Answer 1

Standard marginpars are added by the page breaker so don't work in any kind of box, so a caption or float box on its own would be a enough, a caption inside a float doubly so. Also the caption is written to the list of figures where you probably don't want the note. You could use \caption[..]{...} to have a note-less caption for the lof.

There are implementations of margin notes that do not use the output routine to add the note, for example

\documentclass{article}
\usepackage{marginnote}
\begin{document}
\begin{figure}
XXXXXX
\caption[zzz]{zzzz\marginnote{this}}
\end{figure}
text.. text.. text.. text.. text.. text.. text.. 
text.. text.. text.. text.. text.. text.. text.. 
text.. text.. text.. text.. text.. text.. text.. 
text.. text.. text.. text.. text.. text.. text.. 
text.. text.. text.. text.. text.. text.. text.. 
\end{document}