How can I correct the alignment of an acronym expansion in a table?

Question

I'm using the package acronym to handle acronyms in my document. Unfortunately, when expanding them inside a table they become shifted downwards. Here's a minimal example:

\documentclass{article}
\usepackage[nolist]{acronym}
\begin{document}

\begin{acronym}
  \acro{LONG1}{My extra super long acronym}
  \acro{LONG2}{Yet another very long acronym}
\end{acronym}

\begin{tabular}{ | p{3cm} | p{3cm} |}
  \hline
  \ac{LONG1} & Line 1 \\ \hline
  \ac{LONG2} & Line 2 \\ \hline
\end{tabular}

\end{document}

Ideally, I'd like them to be aligned to the top and not justified. How can I achieve that?

Answer 1

Every time an acronym is used, a line \acronymused{...} is written to the aux file. This introduces a whatsit node which doesn't behave well in vertical mode, like at the start of a p cell. You see the same behaviour here:

\documentclass{article}

\makeatletter
\newcommand*{\foo}{\write\@auxout{\@percentchar hello world}foo}
\makeatother

\begin{document}

\begin{tabular}{ | p{3cm} | p{3cm} | }
\hline
\foo & Line 1 \\ \hline
\leavevmode\foo & Line 2 \\ \hline
\end{tabular}

\end{document}

To avoid the problem you should start horizontal mode before using \ac by writing \leavevmode (or \mbox{}) at the start of the incriminated cells:

\documentclass{article}
\usepackage[nolist]{acronym}
\begin{document}

\begin{acronym}
  \acro{LONG1}{My extra super long acronym}
  \acro{LONG2}{Yet another very long acronym}
\end{acronym}

\begin{tabular}{ | p{3cm} | p{3cm} | }
  \hline
  \leavevmode\ac{LONG1} & Line 1 \\ \hline
  \leavevmode\ac{LONG2} & Line 2 \\ \hline
\end{tabular}

\end{document}

The acronym package should either start its typesetting macros with \leavevmode, or move the \write after the acronym has been typeset.