Unexpected vertical skip amount at end of tabularx environment

Question

I am trying to get my columns to line up on a grid, a task which is sometimes tricky in LaTeX. (See this article for some tips I'm using.)

Everything is working great except this one situation: at the end of my tabularx environment, TeX is skipping slightly less space than it ought to. Notice how the lines are perfectly aligned until the end of the tabular environment in number 9006, and then a couple of points too high for the remainder of the page:

From my reading of the code, it ought to be inserting a \par for the empty line, which skips 0pt, then skipping 1\baselineskip of vertical space at the start of the \itemheading, resulting in an integral multiple of \baselineskip vertical space. That's what's happening for the paragraphs that aren't adjacent to tabular environments.

I'm sure that there is some length (\aftertabularxskip?) that I have failed to set, but I cannot for the life of me find any information on how vertical spacing works before and after a tabular environment, except that tables are treated as boxes, which doesn't help much because it still doesn't explain how the box is sized.

Code for example above (my actual document has a much cleaner set of macros, but I inlined almost all of them for this example to rule out issues with the macros):

% I'm using memoir for this document, but the same issue occurs if
% using the book class and the tabularx package
\documentclass[openany, letter]{memoir}

\usepackage[utf8]{inputenc}
\usepackage{mathpazo}

% the following three lines don't change the problem, it's just easier to see the issue when the page is formatted
\usepackage[inner=1.3in, outer=1.2in, top=54pt, bottom=71pt, columnsep=12pt, headsep=10pt, headheight=12pt, marginparwidth=0.6in, marginparsep=0.2in, heightrounded=true]{geometry}
\usepackage[colaction]{multicol}
\setlength\columnseprule{0.5pt}

\renewcommand\bigskip{\vspace{1\baselineskip}}
\lineskip=0pt
\lineskiplimit=-10pt
\topskip=1\baselineskip
\setlength{\parskip}{0.0pt}
\setlength{\parindent}{15pt}

\newcommand\itemheading[1]{\vspace*{1\baselineskip}\noindent\textbf{#1}\enspace}
\usepackage{lipsum}

\begin{document}
\begin{multicols}{2}

\itemheading{9000}%
\lipsum[1]

\itemheading{9001}%
\lipsum[2]

\itemheading{9002}%
\lipsum[3]

\itemheading{9003}%
\lipsum[4]

\itemheading{9006}\bigskip

\noindent \begin{tabularx}{\linewidth}{r >{\raggedright\arraybackslash}X}
    %\setlength{\extrarowheight}{0pt}
    %\setlength{\extratabsurround}{0pt}
    %\setlength{\aboverulesep}{0pt}
    %\setlength{\belowrulesep}{0pt}
    \emph{X}: & ``This is some test text.''\\
    \emph{Y}: & ``This is some more test text.''\\
    \emph{Z}: & ``This is even more test text.''\\
\end{tabularx}

\itemheading{9007}%
\lipsum[5]

\end{multicols}
\end{document}

Edit: If it's helpful, I made an interesting discovery: the problem does not occur if only one or two lines are present in the tabularx environment. It occurs with three or more, and the error is different depending on how many lines there are. And with four or more, an incorrect amount of space is inserted at the top, too.

Answer 1

TeX keeps lines at constant distance by inserting a glue. If you do e.g.

\documentclass{article}

\parindent=0pt
\begin{document}
\showoutput
aa\\aa\\AA\\..
\end{document}

you will see in the log lines like this:

\glue(\baselineskip) 7.69446
\glue(\baselineskip) 5.16669
\glue(\baselineskip) 10.94446

The glue value differ depending on the height and depth of the adjoining lines: the first ist between aa and aa, the second between aa and AA and the third between A and the dots ..

If you separate the lines with \par the same happens: TeX inserts this glue so that you get uniformed spacing here too:

\documentclass{article}

\parindent=0pt
\parskip=10pt
\begin{document}
\showoutput
aa\par aa\par AA\par ..
\end{document}

When you insert a large object whose height or depth exceeds the default height/depth of a line (and your tabularx is such a large object) this glue is no longer inserted but the fix \lineskip is used instead. This means that the height and depth of the adjoining lines is not longer taken into account: if they contain only dots they will be nearer to the large object as in the case where you have an Aj in it.

So to get alignment again I suggest to

1. use [t] alignment for the tabular (to reduce the problem only to the bottom)
2. Use a strut in the following heading to enforce a normal line height.

The tikz line shows the effect:

\documentclass[openany, letter]{memoir}

\usepackage[utf8]{inputenc}
\usepackage{mathpazo}
\usepackage{tikz}
% the following three lines don't change the problem, it's just easier to see the issue when the page is formatted
\usepackage[inner=1.3in, outer=1.2in, top=54pt, bottom=71pt, columnsep=12pt, headsep=10pt, headheight=12pt, marginparwidth=0.6in, marginparsep=0.2in, heightrounded=true]{geometry}
\usepackage[colaction]{multicol}
\setlength\columnseprule{0.5pt}

\renewcommand\bigskip{\vspace{1\baselineskip}}
\lineskip=0pt
\lineskiplimit=-10pt
\topskip=1\baselineskip
\setlength{\parskip}{0.0pt}
\setlength{\parindent}{15pt}

\newcommand\itemheading[1]{\vspace*{1\baselineskip}\noindent\textbf{\strut #1}\enspace}
\usepackage{lipsum}

\begin{document}
\begin{multicols}{2}

\itemheading{9000}%
\lipsum[1]

\itemheading{9001}%
\lipsum[2]

\itemheading{9002}%
\lipsum[3]

\itemheading{9003}%
\lipsum[4]

\itemheading{9006}\bigskip

\noindent \begin{tabularx}{\linewidth}[t]{r >{\raggedright\arraybackslash}X}
    %\setlength{\extrarowheight}{0pt}
    %\setlength{\extratabsurround}{0pt}
    %\setlength{\aboverulesep}{0pt}
    %\setlength{\belowrulesep}{0pt}
    \emph{X}: & ``This is some test text.''\\
    \emph{Y}: & ``This is some more test text.''\\
    \emph{Z}: & ``This is even more test text.''\\
\end{tabularx}

\itemheading{\tikz[overlay]\draw[red](0,0)--++(-10cm,0);9007}%
\lipsum[5]

\end{multicols}
\end{document}