xltabular breaking booktabs

Question

I want to have a page-filling and -breaking longtable, so I decided to try xltabular. Unfortunately xltabular seems to screw up the booktabs rules in the normal tabular environments, see:

\documentclass{article}
\usepackage{booktabs, xltabular}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tabular}
\begin{document}
\begin{tabular}{ccc}
    \toprule
    col1 & col2 & col3 \\
    \midrule
    row 2: & more and moooooooooooore & test \\
    \bottomrule
\end{tabular}
\end{document}

Not loading xltabular resolves the issue.

Did I miss some package option or some incompatibilities? Is there a better package than xltabular which doesn't break booktabs?

(I am using TeXLive 2019 and compile with lualatex)

---

EDIT (by Skillmon)

I reduced the bug to the following MWE (which doesn't make much sense on its own), it looks like booktabs tries to be smart detecting something, but fails badly.

\documentclass{article}
\usepackage{longtable}
\let\xltabular\relax
\usepackage{booktabs}

\begin{document}
\begin{tabular}{ccc}
    \toprule
    col1 & col2 & col3 \\
    \midrule
    row 2: & more and moooooooooooore & test \\
    \bottomrule
\end{tabular}
\end{document}

It seems to suffice that longtable is loaded and \xltabular is defined (even if it's just \relax).

Answer 1

This bug is confirmed in booktabs.sty version 1.6180339 from 2019/10/08, I don't know whether it is present in earlier versions.

I reported the bug to the maintainer of booktabs. In the meantime, it looks like it could suffice to do the following redefinition of \@BTrule in your preamble:

\documentclass{article}
\usepackage{xltabular}
\usepackage{booktabs}

\makeatletter
\def\@BTrule[#1]{%
  \ifx\longtable\undefined
    \let\@BTswitch\@BTnormal
  \else\ifx\hline\LT@hline
    \nobreak
    \let\@BTswitch\@BLTrule
  \else
     \let\@BTswitch\@BTnormal
  \fi\fi
  \global\@thisrulewidth=#1\relax
  \ifnum\@thisruleclass=\tw@\vskip\@aboverulesep\else
  \ifnum\@lastruleclass=\z@\vskip\@aboverulesep\else
  \ifnum\@lastruleclass=\@ne\vskip\doublerulesep\fi\fi\fi
  \@BTswitch}
\makeatother

\begin{document}
\begin{tabular}{ccc}
    \toprule
    col1 & col2 & col3 \\
    \midrule
    row 2: & more and moooooooooooore & test \\
    \bottomrule
\end{tabular}

\begin{xltabular}{0.7\linewidth}{cXc}
    \toprule
    col1 & col2 & col3 \\
    \midrule
    row 2: & more and moooooooooooore & test \\
    \bottomrule
\end{xltabular}
\end{document}