Vertical spacing of text changes in a macro depending on input

Question

I defined a keynote macro to insert key ideas in my document using varwidth. The document is in \doublespacing and I have defined \singlespacing inside my macro. However, the correct spacing is achieved only for long sentences and not for short text like here.

\documentclass{standalone}
\usepackage{setspace,varwidth,lipsum}

\newcommand{\keynote}[1]{%
    \begin{varwidth}{4cm}%
      {\singlespacing \narrowragged #1}%
    \end{varwidth}%
}

\begin{document}
  \doublespacing
  \hspace{1cm} \keynote{\lipsum[2]}
  \hspace{1cm} \keynote{This note explaining a key idea should go to at least two lines.} \hspace{1cm}
\end{document}

Answer 1

You have to terminate the paragraph, or it will be done when the change in line spacing has already been forgotten because of the }; but since \end{varwidth} ends the paragraph anyway and you're in an environment that forms a group, you can just say

\newcommand{\keynote}[1]{%
   \begin{varwidth}{4cm}
   \singlespacing \narrowragged #1
   \end{varwidth}%
}

Why the example had that peculiarity? Because \lipsum[2] has a \par command inside it. If you used \lipsum*[2], that doesn't issue \par you'd have seen the same problem.

Now you know why: in

\begin{varwidth}{4cm}
{\singlespacing\narrowragged some text to be single spaced}
\end{varwidth}

the paragraph "some text to be single spaced" will be built when the } has already undone the \singlespacing\narrowragged setting. Without the inner braces it's OK.

Answer 2

Use the spacing environment of the 'setspace' package.

\documentclass{standalone}
\usepackage{setspace,varwidth,lipsum}

\newcommand{\keynote}[1]{%
    \begin{varwidth}{4cm}%
      \begin{spacing}{1}
        {\narrowragged #1}%
      \end{spacing}
    \end{varwidth}%
}

\begin{document}
 \doublespacing
  \hspace{1cm} \keynote{\lipsum[2]} \hspace{1cm} \keynote{This note
    explaining a key idea should go to at least two lines.}
  \hspace{1cm}
\end{document}