Wrong output of subscripts while using Hendrik Vogt's "\widebar"

Question

When I try to use Hendrik Vogt's "\widebar" from Can I get a \widebar without using the mathabx package?, it happens that the output of

\widebar{\mathcal{I}}_{x}=\widebar{\mathcal{I}_{x}}

becomes as follows:

.

The inside index "x" changes. But if I add an extra "{ }" like \widebar{{\mathcal{I}}_{x}}, I will get the right form

I have no idea how to fix the code to make it work without extra "{ }". Could anyone help please?

Answer 1

You get the same output as

\mathcal{I_x}

just with overline above it. You can use

\widerbar{{\mathcal{I}}_{x}}

to fix it.

I reproduce the behavior in the example code below to show that, apart from the wrong symbol being used, the output is the same as with \overline.

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\let\save@mathaccent\mathaccent
\newcommand\if@single[3]{%
  \setbox0\hbox{${\mathaccent"0362{#1}}^H$}%
  \setbox2\hbox{${\mathaccent"0362{\kern0pt#1}}^H$}%
  \ifdim\ht0=\ht2 #3\else #2\fi
}
%The bar will be moved to the right by a half of \macc@kerna, which is computed by amsmath:
\newcommand\rel@kern[1]{\kern#1\dimexpr\macc@kerna}
%If there's a superscript following the bar, then no negative kern may follow the bar;
%an additional {} makes sure that the superscript is high enough in this case:
\newcommand\widebar[1]{@ifnextchar^{{\wide@bar{#1}{0}}}{\wide@bar{#1}{1}}}
%Use a separate algorithm for single symbols:
\newcommand\wide@bar[2]{\if@single{#1}{\wide@bar@{#1}{#2}{1}}{\wide@bar@{#1}{#2}{2}}}
\newcommand*\wide@bar@[3]{%
  \begingroup
  \def\mathaccent##1##2{%
%Enable nesting of accents:
    \let\mathaccent\save@mathaccent
%If there's more than a single symbol, use the first character instead (see below):
    \if#32 \let\macc@nucleus\first@char \fi
%Determine the italic correction:
    \setbox\z@\hbox{$\macc@style{\macc@nucleus}{}$}%
    \setbox\tw@\hbox{$\macc@style{\macc@nucleus}{}{}$}%
    \dimen@\wd\tw@
    \advance\dimen@-\wd\z@
%Now \dimen@ is the italic correction of the symbol.
    \divide\dimen@ 3
    @tempdima\wd\tw@
    \advance@tempdima-\scriptspace
%Now @tempdima is the width of the symbol.
    \divide@tempdima 10
    \advance\dimen@-@tempdima
%Now \dimen@ = (italic correction / 3) - (Breite / 10)
    \ifdim\dimen@>\z@ \dimen@0pt\fi
%The bar will be shortened in the case \dimen@<0 !
    \rel@kern{0.6}\kern-\dimen@
    \if#31
      \overline{\rel@kern{-0.6}\kern\dimen@\macc@nucleus\rel@kern{0.4}\kern\dimen@}%
      \advance\dimen@0.4\dimexpr\macc@kerna
%Place the combined final kern (-\dimen@) if it is >0 or if a superscript follows:
      \let\final@kern#2%
      \ifdim\dimen@<\z@ \let\final@kern1\fi
      \if\final@kern1 \kern-\dimen@\fi
    \else
      \overline{\rel@kern{-0.6}\kern\dimen@#1}%
    \fi
  }%
  \macc@depth@ne
  \let\math@bgroup@empty \let\math@egroup\macc@set@skewchar
  \mathsurround\z@ \frozen@everymath{\mathgroup\macc@group\relax}%
  \macc@set@skewchar\relax
  \let\mathaccentV\macc@nested@a
%The following initialises \macc@kerna and calls \mathaccent:
  \if#31
    \macc@nested@a\relax111{#1}%
  \else
%If the argument consists of more than one symbol, and if the first token is
%a letter, use that letter for the computations:
    \def\gobble@till@marker##1\endmarker{}%
    \futurelet\first@char\gobble@till@marker#1\endmarker
    \ifcat\noexpand\first@char A\else
      \def\first@char{}%
    \fi
    \macc@nested@a\relax111{\first@char}%
  \fi
  \endgroup
}
\makeatother
\linespread{1.44}
\begin{document}
$\mathcal{I_x}$
$\widebar{\mathcal{I}}{x}=\widebar{\mathcal{I}{x}}$
\quad
$\widebar{\mathcal{I}}_{x}=\overline{\mathcal{I_x}}$
$\widebar{\mathcal{I}}{x}=\widebar{{\mathcal{I}}{x}}$
\quad
$\widebar{\mathcal{I}}{x}=\overline{\mathcal{I}{x}}$
\end{document}

In the third line you can see that the output of \widebar{{\mathcal{I}}_{x}} and of \overline{\mathcal{I}_{x}} are identical.

Compare with the following, based on https://tex.stackexchange.com/a/364929/4427

\documentclass{article}
\usepackage{amsmath}
\DeclareFontFamily{U}{mathx}{\hyphenchar\font45}
\DeclareFontShape{U}{mathx}{m}{n}{ <-> mathx10 }{}
\DeclareSymbolFont{mathx}{U}{mathx}{m}{n}
\DeclareFontSubstitution{U}{mathx}{m}{n}
\DeclareMathAccent{\widebar}{\mathalpha}{mathx}{"73}
\linespread{1.44}
\begin{document}
$\widebar{\mathcal{I}}{x}=\widebar{\mathcal{I}{x}}$
$\widebar{\mathcal{I}}{x}=\overline{\mathcal{I}{x}}$
\end{document}