Unable to use dvisvgm when l3draw drawing contains scoped box

Question

I found a strange issue when experimenting with dvisvgm trying to create SVG files from DVI files.

I found that if I append a box (or a coffin) to a drawing created with the l3draw package and this box (or coffin) is inside a scope, the DVI created via a run of latex cannot be converted to an SVG via dvisvgm. But if this box (of coffin) is not inside a scope, it works.

Consider the following MWE:

\documentclass[border=10pt]{standalone}
\usepackage{l3draw}

\begin{document}
    
    \ExplSyntaxOn
    \draw_begin:
    
        \draw_path_circle:nn { 0cm , 0cm } { 1cm }
        \draw_path_use_clear:n { stroke }
        
        \draw_scope_begin:
        \hbox_set:Nn \l_tmpa_box {foo}
        \draw_box_use:N \l_tmpa_box
        \draw_scope_end:
    
    \draw_end:
    \ExplSyntaxOff
    
\end{document}

I run latex and then dvisvgm and the error PostScript error: stackunderflow in restore will occur. If I comment out the scope, the file is converted without problems and the circle and the text in the box show up in resulting SVG.

I am unsure where to locate the problem: With the l3draw package or with dvisvgm (or somewhere else altogether).

Answer 1

This seems to be a bug in the dvips backend of l3draw.

\draw_scope_begin: and \draw_scope_end: create PS code that looks like this:

@beginspecial
...
save
@endspecial
...
@beginspecial
restore
@endspecial

The PS operator save puts a save object on the operand stack that holds information about the current state of the PS interpreter. restore, on the other hand, expects a save object on the stack to restore the corresponding memory state.

However, @endspecial which is a PS procedure defined by dvips, removes the save object created by save. Therefore, there's nothing left on the stack when restore is executed and that's why the PS interpreter stops with stackunderflow in restore.

Answer 2

You are using the dvips back end, with the dvisvgm back end you get no error

\documentclass[border=10pt]{standalone}
\ExplSyntaxOn
\sys_load_backend:n {dvisvgm}
\ExplSyntaxOff
\usepackage{l3draw}

\begin{document}
    
    \ExplSyntaxOn
    \draw_begin:
    
        \draw_path_circle:nn { 0cm , 0cm } { 1cm }
        \draw_path_use_clear:n { stroke }
        
        \draw_scope_begin:
        \hbox_set:Nn \l_tmpa_box {foo}
        \draw_box_use:N \l_tmpa_box
        \draw_scope_end:
    
    \draw_end:
    \ExplSyntaxOff
    
\end{document}

although I just get the text