Prevent unnecessary longtable widows

Question

When the remaining rows of a longtable fit exactly onto the current page, longtable will instead create a widow by printing a footer on the current page and moving the last row onto the next page (with repeated header and footer).

This can be prevented by printing the last row as part of the last footer, but I am wondering why this is happening in the first place, and whether this could perhaps be fixed in the longtable package. (Conceptually, I would expect the algorithm to only start a new page after it has determined that the remaining contents do not fit onto the current page.)

\documentclass{article}
\usepackage{booktabs}
\usepackage{longtable}
\begin{document}
\begin{longtable}{l}
\caption{Caption}\
\toprule
Column heading\
\midrule
\endfirsthead
\caption[]{Caption} \
\toprule
Column heading\
\midrule
\endhead
\midrule
Continued on next page\
\midrule
\endfoot
\bottomrule
\endlastfoot
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\end{longtable}
\end{document}

Answer 1

longtable really can't look ahead, the whole point of it is to defer page breaking to the asynchronous output routine as for normal page breaking, so it doesn't have control over when page breaking decisions happen, the output routine jumps in when it jumps. It is measuring the space required using the vertical size of the footbox, it never checks that lastfoot is smaller, it could check that but I'm not sure what it could do.

That's the thing about tex: it is designed for the memory of a machine in 1979, it doesn't have the document in memory and might have written the last page to the pdf before \end{longtable} has even been read from the file system. longtable can't look ahead and it can't go back and change decisions (but it could typeset 1000 row tables in 1990 on a machine with less than 1Mb of memory....)

As shown in other answers newer packages make different assumptions and grab the whole table first. That gives more flexibility but is a complete redesign, not a feature that can simply be added

Answer 2

If you willing to use longtblr of tabularray package instead of longtable, then in your particular case table can be fit in one page even with one more row in table body:

(red lines show page layout)

Beside this the table code is (far) more concise and simpler:

\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[latin, english]{babel}
%---------------------------------------------------------------%
\usepackage{tabularray}
\UseTblrLibrary{booktabs}
\begin{document}
\begin{longtblr}[
caption = {Caption}
                ]{colspec={X[c]},
                  rowsep=0pt,
                  row{1}={font=\bfseries},
                 rowhead=1}
    \toprule
Column heading\
    \midrule
%
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%
one more table row\
    \bottomrule
    \end{longtblr}
\end{document}

In the case that table is longer (has more rows or rows with multi lines text), it will split over page and automatic insert (continued) to caption on the next page and ˙continued on next page` at table bottom on the first page