"Extra }, or forgotten \endgroup" when combining "fixltx2e" and "preview" packages

Question

In a two-column document with figure* floats, where the preview package is set up to preview floats, compilation seems to fail if the fixltx2e package is loaded:

\documentclass[twocolumn]{article}

\usepackage{fixltx2e}
\usepackage[active,floats]{preview}

\begin{document}
  \begin{figure*}Test\end{figure*}
\end{document}

The first error message seen reads:

! Extra }, or forgotten \endgroup.
\@endfloatbox ...pagefalse \outer@nobreak \egroup 
                                                  \color@endbox 
l.7   \begin{figure*}Test\end{figure*}

? 

The error vanishes if fixltx2e is not loaded or if the figure environment is used.

My goal is to "preview" all figures in a two-column document, so that \linewidth is set up correctly within the environments. What am I doing wrong? Is there a way to "fix" this while retaining fixltx2e? Is this package needed at all in a recent LaTeX installation?

Answer 1

The error happens when \pr@\end@dblfloat is being executed, which uses \end@dblfloat.

Let's compare the definitions of \end@dblfloat in the kernel and in fixltx2e:

latex.ltx

\def\end@dblfloat{%
\if@twocolumn
  \@endfloatbox
  \ifnum\@floatpenalty <\z@
    \@largefloatcheck
    \@cons\@dbldeferlist\@currbox
  \fi
    \ifnum \@floatpenalty =-\@Mii \@Esphack\fi
\else
  \end@float
\fi
}

fixltx2e.sty

\def\end@dblfloat{%
\if@twocolumn
  \@endfloatbox
  \ifnum\@floatpenalty <\z@
    \@largefloatcheck
    \global\dp\@currbox1sp %
     \expandafter\@gobble\end@float
  \fi
\else
  \end@float
\fi
}

As you see, the \@largefloatcheck command is followed by two quite different paths and this confuses preview.sty that relies on the kernel definition.

I don't see any way around this, unless preview.sty is updated to consider the case when fixltx2e is loaded.