Line break command ignored within frame in beamerarticle

[ysalmon]

I have a tblr with a multiline cell, but the \\ line breaking command seems ignored when in a frame environment in a beamerarticle :

\documentclass{article}
\usepackage{beamerarticle}
\usepackage{tabularray}

\begin{document}
\begin{frame}
\begin{tblr}{hlines, vlines,
             columns={halign=c, valign=m, font=\scriptsize, colsep=4pt},
             column{1}={wd=4em}}
{$L_1.L_2$ \\ \tiny non vides}  \\
\end{tblr}
\end{frame}
\end{document}

gives

If I remove the frame environment, I correctly get

Of course the frame is there because my real example is from a larger presentation of which I am doing the article counterpart. Adding [fragile] does not help.

In the beamer version of the document, the line break is where I want it, but that may be by chance (it seems the line break happens "naturally" where LaTeX wants to put it, and not where I request it).

[ponte-vecchio]

I suspect there's an interplay between the value of the designated wd, and the size of the font.

Consider the following example that explores different font sizes and whether the wd is given or not:

Details

\documentclass{article}
\usepackage{beamerarticle}
\usepackage{tabularray}

\begin{document}
  \begin{frame}
    \begin{tblr}{
        hlines, vlines,
        columns={c,m,colsep=4pt, font=\small}
    }
      {$L_1 \cdot L_2$ \\ {\tiny non vides}}  \\
    \end{tblr}
    \begin{tblr}{
        hlines, vlines,
        columns={c,m,colsep=4pt, font=\footnotesize}
      }
      {$L_1 \cdot L_2$ \\ {\tiny non vides}}  \\
    \end{tblr}
    \begin{tblr}{
        hlines, vlines,
        columns={c,m,colsep=4pt, font=\tiny}
      }
      {$L_1 \cdot L_2$ \\ {\tiny non vides}}  \\
    \end{tblr}
  \end{frame}
  
  %---
  
  \begin{frame}
    \begin{tblr}{
        hlines, vlines,
        columns={c,m,4em,colsep=4pt, font=\small}
      }
      {$L_1 \cdot L_2$ \\ {\tiny non vides}}  \\
    \end{tblr}
    \begin{tblr}{
        hlines, vlines,
        columns={c,m,4em,colsep=4pt, font=\footnotesize}
      }
      {$L_1 \cdot L_2$ \\ {\tiny non vides}}  \\
    \end{tblr}
    \begin{tblr}{
        hlines, vlines,
        columns={c,m,4em,colsep=4pt, font=\tiny}
      }
      {$L_1 \cdot L_2$ \\ {\tiny non vides}}  \\
    \end{tblr}
  \end{frame}
\end{document}

The following output is given:

[muzimuzhi]

The article mode of beamer ignores \\, see texdoc beamer, sec. 21.2.1 "Starting the Article Mode":

There is one command whose behavior is a bit special in article mode: The line break command \\. Inside frames, this command has no effect in article mode, except if an overlay specification is present. Then it has the normal effect dictated by the specification. The reason for this behavior is that you will typically inserts lots of \\ commands in a presentation in order to get control over all line breaks. These line breaks are mostly superfluous in article mode. If you really want a line break to apply in all versions, say \\<all>. Note that the command \\ is often redefined by certain environments, so it may not always be overlay-specification-aware. In such a case you have to write something like \only<presentation>{\\}.

In tabularray,

• the initial setting halign=j lets \\ to \@normalcr (see \__tblr_halign_command_j:, then \TblrAlignBoth), which is not altered by beamerarticle, so with haligh=j, \\ works inside tblr cells;
• by setting halign=c (OP's columns={c, ...} is a shortcut), \\ is let to \@centercr (see \__tblr_halign_command_c:, then \TblrAlignCenter and \centering), which unfortunately is altered by beamerarticle, so with haligh=c, \\ is ignored inside tblr cells. One needs to use \\<all> or setting something like columns={preto={\let\\=\beamer@origbreakcenter}}.

tabularray/tabularray.sty

Lines 171 to 175 in 6fe650f

	%% Some commands for horizontal alignment
	\cs_new_eq:NN \__tblr_halign_command_j: \TblrAlignBoth
	\cs_new_eq:NN \__tblr_halign_command_l: \TblrAlignLeft
	\cs_new_eq:NN \__tblr_halign_command_c: \TblrAlignCenter
	\cs_new_eq:NN \__tblr_halign_command_r: \TblrAlignRight

tabularray/tabularray.sty

Lines 56 to 69 in 6fe650f

	\NewDocumentCommand \TblrAlignBoth { }
	{
	\let \\ = \@normalcr
	\leftskip = \z@skip
	\@rightskip = \z@skip
	\rightskip = \@rightskip
	\parfillskip = \@flushglue
	}
	\NewDocumentCommand \TblrAlignLeft { } { \raggedright }
	\NewDocumentCommand \TblrAlignCenter { } { \centering }
	\NewDocumentCommand \TblrAlignRight { } { \raggedleft }

Example

\documentclass{article}
\usepackage{beamerarticle}
\usepackage{tabularray}

\NewDocumentCommand{\testTblrLinebreak}{mm}{
  \begin{tblr}{
      hlines, vlines,
      columns={#1}
    }
    {$L_1 \cdot L_2$ #2 {\tiny non vides}}  \\
  \end{tblr}
  \parbox{15em}{\tiny\ttfamily\frenchspacing\raggedright\sloppy
    options: "\detokenize{columns={#1}}" \newline
    linebreak command: "\detokenize{#2}"
  }
}

\begin{document}
\begin{frame}
  % failing cases
  \testTblrLinebreak{halign=c, 8em}{\\}

  % working cases
  \testTblrLinebreak{halign=j, 8em}{\\} % `j` is the default haligh

  \testTblrLinebreak{halign=c, 8em}{\\<all>}

  \makeatletter
  \def\tblrResetCentercr{%
    \let\\=\beamer@origbreakcenter
  }
  \makeatother
  \testTblrLinebreak{halign=c, 8em, preto=\tblrResetCentercr}{\\}
\end{frame}
\end{document}

Output

[ysalmon]

Thank you so much for this in-depth investigation !

Maybe this setting should be done by tblarray, as the rationale of beamerarticle's behaviour with line breaks does not apply within table cells.