Problem with vertical centering of cells of a same row of a longtable

Question

This question follows up a previous question: Vertically centering subtable in cells of a main longtable.

I am using the code:

\documentclass{article}
\usepackage{longtable}
\usepackage{array}
\usepackage{xparse}
\usepackage{lipsum}

\newcolumntype{L}[1]{>{\raggedright\arraybackslash}p{#1}}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}
\newcolumntype{R}[1]{>{\raggedleft\arraybackslash}p{#1}}

\NewDocumentCommand{\TB}{O{c} m O{c}}{%
    \begin{tabular}[#1]{@{}#3@{}}#2\end{tabular}
}

\renewcommand*{\arraystretch}{1.15}

\begin{document}
    \begin{longtable}{|L{6cm}|C{2cm}|C{2cm}|}
        \caption{Your table caption} \label{tab:my_label} \\
        \hline
        header 1  & header 2    & header 3 \\
        \hline
        \endhead
        \lipsum[1]      & \TB{a1\\a2} & \TB{b1\\b2\\b3} \\
        \hline
        row2      & a3          & \TB[t]{b4\\b5} \\
        \hline
        row3      & a4          & \TB[b]{b6\\b7} \\
        \hline
    \end{longtable}
\end{document}

Unfortunately, the cells of the first row are not being vertically centered:

Could you please help me? Thanks!

Answer 1

I suggest you switch the underlying column type of the L, C, and R column types from p to m. (m is mnemonic for "middle", I suppose.)

\documentclass{article}
\usepackage{longtable,array,xparse,lipsum}

\newcolumntype{L}[1]{>{\raggedright\arraybackslash}m{#1}}
\newcolumntype{C}[1]{>{\centering\arraybackslash}m{#1}}
\newcolumntype{R}[1]{>{\raggedleft\arraybackslash}m{#1}}

\NewDocumentCommand{\TB}{O{c} m O{c}}{%
    \begin{tabular}[#1]{@{}#3@{}}#2\end{tabular}
}

\renewcommand*{\arraystretch}{1.15}

\begin{document}
    \begin{longtable}{|L{6cm}|C{2cm}|C{2cm}|}
        \caption{Your table caption} \label{tab:my_label} \\
        \hline
        header 1  & header 2    & header 3 \\
        \hline
        \endhead
        
        \lipsum[1][1-8] & \TB{a1\\a2} & \TB{b1\\b2\\b3} \\
        \hline
    \end{longtable}
\end{document}

Answer 2

Like this?

(red lines indicate page layout)

By use longtblr of tabularray package, code for MWE is simple and concise:

\documentclass{article}
%--------------- show page layout. don't use in a real document!
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%
\usepackage{lipsum}                             % for dummy text
%---------------------------------------------------------------%
\usepackage{ragged2e}
\usepackage{tabularray}
\SetTblrStyle{contfoot}{font=\small\itshape}

\begin{document}
    \begin{longtblr}[
caption = {Your table caption},
  label = {tab:my_label}
                    ]{hlines, vlines,
                      colspec = {X[m, cmd={\RaggedRight\hspace{0pt}}] *{2}{Q[c, m]}},
                      rowhead = 1
                      }
header 1    &    header 2   &   header 3        \\
% table body
\lipsum[1]  &   {a1\\a2}    &   {b1\\b2\\b3}    \\
row2        &   a3          &   {b4\\b5}        \\
row3        &   a4          &   {b6\\b7}        \\
    \end{longtblr}
\end{document}

Edit:

• As you can see, I took a liberty and a little bit redesign your table that according to my opinion make table more pretied:
  • last two columns has natural width,
  • width of the first column occupy the rest widths of the \textwidth.
• Contents in cells are vertical centered by use of the m option in the X and Q columns specifications.
• For more details about tabularray package you should consult package documentation which contain concise explanation with a plenty examples. For your problem may be interesting the following:
• beside standard tabular aligning c (horizontal center), l, and r introduce f (at foot of cell), h (at top of cell) and m (at middle, vertical centered), which can be also used as options in Q and X cells (later is known from tabularx package)
• for manual breaking cells' contents is introduced simple syntax {<first line>\\<second line>\\ ... }
• for multi column and multirow cells are defined SetCell[c=<number od columns>]{<aligning of contents} cell content and SetCell[r=<number od columns>]{<aligning of contents} cell content
• Other useful features are:
• automatic adding continue to long table captions on the next pages and `table footmark continue on next page
• repeating of rows, which contains column headers, is determined by rowhead = <number> instruction in the table preamble.