Strange alignment problems in longtable

Question

I have written a few environments for formatting use cases. For some reason I am having strange alignment issues in my longtable. I think that there is some invisible placeholder in there messing it all up, but after several hours, and after removing all unnecessary spaces, I can't find it.

Here are my environments:

\newenvironment{ucflowofevents}{%
\newcommand{\event}[1]{\item{##1}}%
\begin{enumerate}
}{%
\end{enumerate}
}

\newenvironment{ucentryconditions}{%
\newcommand{\cond}[1]{\item{##1}}%
\begin{itemize}
}{%
\end{itemize}
}

\newenvironment{ucexitconditions}{%
\newcommand{\cond}[1]{\item{##1}}%
\begin{itemize}
}{%
\end{itemize}
}

\newenvironment{ucrequirements}{%
\newcommand{\req}[1]{\item{##1}}%
\begin{itemize}
}{%
\end{itemize}
}

\newenvironment{pcusecase}[1]{%
    \newcommand{\row}[3][\hline]{##2 & ##3 \\ ##1}
    \newcolumntype{L}{>{\vspace{-1.0\baselineskip}\flushleft\arraybackslash}p{4cm}}%
    \newcolumntype{R}{>{\vspace{-1.0\baselineskip}\raggedright\arraybackslash}b{10cm}}%
    \newcommand{\giveninitactors}{REQUIRED!!!}%
    \newcommand{\givenevents}{REQUIRED!!!}%
    \newcommand{\givencommswith}{}%
    \newcommand{\givenentries}{}%
    \newcommand{\givenexits}{}%
    \newcommand{\givenreqs}{}%
    \newcommand{\initactors}[1]{\gdef\giveninitactors{##1}}%
    \newcommand{\commswith}[1]{\gdef\givencommswith{##1}}%
    \newcommand{\events}[1]{\gdef\givenevents{\begin{ucflowofevents}##1\end{ucflowofevents}}}%
    \newcommand{\entries}[1]{\gdef\givenentries{\begin{ucentryconditions}##1\end{ucentryconditions}}}%
    \newcommand{\exits}[1]{\gdef\givenexits{\begin{ucexitconditions}##1\end{ucexitconditions}}}%
    \newcommand{\reqs}[1]{\gdef\givenreqs{\begin{ucrequirements}##1\end{ucrequirements}}}%
    \let\givencommswith\empty%
    \let\givenentries\empty%
    \let\givenexits\empty%
    \let\givenreqs\empty%
    \centering%
    \begin{longtable}{|LR|}\hline %
        \row{Use Case:}{#1}
}{%
        \row{Initiating Actors:}{\giveninitactors}%
        \ifx\givencommswith\empty\else \row[]{Communicates With:}{\givencommswith} \fi %\hline %
        \hline\row{Flow of Events:}{\givenevents} %
        \ifx\empty\givenentries\else \row{Entry Conditions:}{\givenentries} \fi %
        \ifx\empty\givenexits\else \row{Exit Conditions:}{\givenexits} \fi %
        \ifx\empty\givenreqs\else \row{Quality Requirements:}{\givenreqs} \fi%
    \end{longtable}%
}

The first four just set up lists for the flow of events, entry and exit conditions, and quality requirements. But pcusecase sets up the longtable.

Notice the \newcolumntype definitions:

    \newcolumntype{L}{>{\vspace{-1.0\baselineskip}\flushleft\arraybackslash}p{4cm}}%
    \newcolumntype{R}{>{\vspace{-1.0\baselineskip}\raggedright\arraybackslash}b{10cm}}%

What I really want is for them both to be top aligned, but I specified b for the right-hand (R) column because it exaggerates what happens. Here is my use case:

\begin{pcusecase}{TestUseCase}%
    \initactors{Rider, Admin}%
    \commswith{GPSService}%
    \events{\item{Event 1}%
                    \item{Event 2}}%
    \entries{\cond{Entry Condition 1}%
                     \cond{Entry Condition 2}}%
    \exits{\cond{Exit Condition 1}%
                 \cond{Exit Condition 1}}%
    \reqs{\req{Requirement 1}%
                \req{Requirement 2}
                \req{Requirement 3}}%
\end{pcusecase}

which gives this output:

Notice that for the first three rows, the right-hand cells are bottom-aligned like they should be, but for the remaining rows, the left cells are bottom-aligned and the right cells are top-aligned.

If I specify that left should be bottom-aligned, and the right top-aligned with

    \newcolumntype{L}{>{\vspace{-1.0\baselineskip}\flushleft\arraybackslash}b{4cm}}%
    \newcolumntype{R}{>{\vspace{-1.0\baselineskip}\raggedright\arraybackslash}p{10cm}}%

I get this:

The left is somehow top-aligned and the right-bottom aligned, sort of. The weirdest one, and probably the one that is going to clue somebody in, is when I specify that both should be bottom aligned and get this:

In all three versions, the alignment in the three upper-left cells didn't change. I've been staring for hours. I need some fresh eyes on this. Your help is greatly appreciated...

Answer 1

Here's a demonstration of how the standard vertical alignment options work:

\documentclass{article}
\usepackage{tikz}
\begin{document}
  \newcommand\demobox[1]{%
  \fbox{%
    \begin{minipage}[#1]{.25\textwidth}
      \tikz[baseline=(n.base)]{%
        \node (n) [inner sep=0pt, outer sep=0pt, text width=\linewidth] {Top};
        \draw [red] (n.west |- n.base) -- (n.east |- n.base);}
      \par
      This is a box with some content which should be longer than can be accommodated on one line.
      \par
      \tikz[baseline=(n.base)]{%
        \node (n) [inner sep=0pt, outer sep=0pt, anchor=base, text width=\linewidth] {Bottom};
        \draw [red] (n.west |- n.base) -- (n.east |- n.base);}
    \end{minipage}}}
\demobox{t}
\demobox{b}
\demobox{c}
\end{document}

The red lines mark the baselines of the first and final lines of each minipage. Because the first is aligned with t and the second with b, the baseline of the first line of the first and the baseline of the final line of the second are aligned. (c is just there fore comparison.)

The same thing happens if the alignment is with the baseline of a line of regular text:

This is a line of regular text across the page. Here we go\dots\par
and some text \demobox{t}
and some more \demobox{b}\par

The other complication is that enumerate environments, by default, include vertical spacing before the first and after the last item:

\documentclass{article}
\begin{document}
  \fbox{%
    \parbox[t]{.25\textwidth}{%
      \begin{enumerate}
        \item Item
        \item Item
        \item Item
      \end{enumerate}}} and here is the line of text.
  \fbox{%
    \parbox[t]{.25\textwidth}{%
      Top line of text.\par Next line.\par Bottom line.}}

  \fbox{%
    \parbox[b]{.25\textwidth}{%
      \begin{enumerate}
        \item Item
        \item Item
        \item Item
      \end{enumerate}}} and here is the line of text.
  \fbox{%
    \parbox[b]{.25\textwidth}{%
      Top line of text.\par Next line.\par Bottom line.}}
\end{document}