empheq amsart enumerate error

Question

I have started using the empheq package and run into some issues. The following compiles normally:

documentclass[fleqn]{article}
usepackage{empheq}
usepackage{xcolor}
newcommand*mybluebox[1]{colorbox{blue!20}{hspace{1em}#1hspace{1em}}}

begin{document}
begin{enumerate}
item We have
begin{empheq}[box=mybluebox]{equation}
f(x)=g(x)
end{empheq}
begin{empheq}[box=mybluebox]{align}
f(x)=f(x)=g(x)
end{empheq}
item etc.
end{enumerate}
end{document}

Result:

On the other hand, in the amsart class, there is a glitch with the align environment and empheq (which does not seem to appear with the equation environment), namely, replacing the class with

documentclass[fleqn]{amsart}

messes up the empheq environment. Does anybody have an idea of how to fix it?

daleif · Answer

Not really an answer, but need more space

Update 2020/03/18: Updated fix. Though this does not work with the flalign env

documentclass[fleqn]{amsart}
usepackage{empheq}
usepackage{xcolor}
newcommand*mybluebox[1]{colorbox{blue!20}{hspace{1em}#1hspace{1em}}}

usepackage{xpatch}
MHInternalSyntaxOn
defEQ_initialize:{%
  m@th
  MH_let:NwNSetTagPlusEndMarkEQ_tag_plus_endmark:
  MH_let:NwNSetOnlyEndMarkEQ_only_endmark:
  l_EQ_temp_linewidth_dimlinewidth
  @tempdimaz@
  begingroup
  ifdefinedfullwidthdisplayrelax
    fullwidthdisplay
    global@tempdimadisplaywidth
  fi
  endgroup
  ifdim@tempdima>linewidthrelax
    l_EQ_temp_linewidth_dim@tempdima
  fi
  @tempdimaz@
  g_EQ_toprow_height_dimz@
  g_EQ_bottomrow_depth_dimz@
  g_EQ_toptag_height_dimz@
  g_EQ_bottomtag_depth_dimz@
  g_EQ_widesttag_dimz@
  xdefg_EQ_latest_nonzerodepth_row_fint{z@}
  edefEQ_restore_tex_delimiter:{%
    delimiterfactorthedelimiterfactor
    delimitershortfallthedelimitershortfallrelax}%
  g@addto@macroEQ_complete_mathdisplay:
                EQ_restore_tex_delimiter:
  delimitershortfallEmphEqdelimitershortfall
  delimiterfactorEmphEqdelimiterfactor
  MH_let:NwNintertext@EQ_intertext:
  EQ_displaybreak:}

% trick to see what empheq is typesetting
%defEQ_use_mathdisplay:{%
%  EQ_outerbox:{fbox{usebox{EQ_mathdisplay_box}}}}

MHInternalSyntaxOff

begin{document}
begin{enumerate}
item We have
begin{empheq}[box=mybluebox]{equation}
f(x)=g(x)
end{empheq}
begin{empheq}[box=mybluebox]{align}
f(x)=f(x)=g(x)
end{empheq}
item no empheq
  begin{equation}
    f(x)=g(x)
  end{equation}
  begin{align}
    f(x)=f(x)=g(x)
end{align}
end{enumerate}
end{document}

empheq amsart enumerate error

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