Why does a paragraph ending on a line break add an extra parskip?

Question

Consider the following example:

\documentclass{article}

\newenvironment{eparskip} {
  \parindent0pt
  \setlength{\parskip}{1em}
}{
  \vspace{1em}
}

\begin{document}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not have weird spacing issues.
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not have weird spacing issues.
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not
The paragraphs in this block will have single space and no indent, and will not
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not have weird spacing issues.
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not have weird spacing issues.
\end{eparskip}

\end{document}

which produces:

I know there's a stigma against adjusting the parskip parameter, but this behavior also happens with the parskip package itself. Is there a solution to this?

Answer 1

The third example ends with

<space><space>\vspace{1em}<bookkeeping business><space>\par

where the <bookkeeping business> is done by \end{eparskip} and is irrelevant to the issue.

The relevant thing is that the horizontal spaces before \vspace are not removed and make for a third line in the paragraph.

Where do they come from? The first one is from the end-of-line after not; the second is from the end-of-line before \vspace in the environment's definition. There's another one, coming from the end-of-line after \end{eparskip}, which comes before the \par generated by the blank line. This one is removed by \par, the others aren't.

\documentclass{article}

\newenvironment{eparskip}
 {\par\setlength{\parindent}{0pt}\setlength{\parskip}{1em}}
 {\par\vspace{1em}}

\begin{document}

\begin{eparskip}
The paragraphs in this block will have single space and no 
indent, and will not have weird spacing issues.
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no 
indent, and will not have weird spacing issues.
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not
The paragraphs in this block will have single space and no indent, and will not
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no 
indent, and will not have weird spacing issues.
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no 
indent, and will not have weird spacing issues.
\end{eparskip}

\end{document}

Be careful with line endings in definitions; also ensure that \vspace is issued between paragraphs, unless you have special needs.

Answer 2

the extra lines contain the spurious spaces you have added in the environment definition. Remove them and you get:

\documentclass{article}

\newenvironment{eparskip} {%
  \parindent0pt
  \setlength{\parskip}{1em}%
}{%
  \par
  \vspace{1em}
}

\begin{document}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not have weird spacing issues.
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not have weird spacing issues.
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not
The paragraphs in this block will have single space and no indent, and will not
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not have weird spacing issues.
\end{eparskip}

\begin{eparskip}
The paragraphs in this block will have single space and no indent, and will not have weird spacing issues.
\end{eparskip}

\end{document}