How can I improve alignment of the numerator and denominator when vector norm bars along with subscripts throw things off?

Question

Here's the really bad case:

With a red bar to show what is bothering me in particular:

Here's a simpler case where the issue is a non-issue, but perhaps one can get a hint of why it starts to get bad:

Here's some minimum code to produce the problem examples:

\documentclass{article}

\usepackage{amsmath}
\usepackage{physics}

\newcommand{\mycommand}[1]{\ensuremath{{#1}_{p}}}

\renewcommand{\vec}[1]{\ensuremath{\mathbf{#1}}}
\newcommand{\normv}[2][p]{%
    \ensuremath{\norm{\vec{#2}}_{#1}}%
}

\begin{document}
    \begin{equation*}
        0 = \frac{x}{\mycommand{x}}
    \end{equation*}

    \begin{equation*}
        \vec{x} = \frac{\vec{x}}{\normv{x}}
    \end{equation*}
\end{document}

I got the following bit of code from the answer to this question:

\newcommand{\alignedvfrac}[2]{%
    \setbox0\hbox{$#1$}        % put the numerator in box0
    \dimen0=\wd0               % measure box0
    \setbox1\hbox{$#2$}        % put the denominator in box1
    \dimen1=\wd1               % measure box1
    \ifdim\wd0<\wd1            % if box0 is narrower than box1
    \dfrac{#1\hfill}{#2}   % put \hfill in the numerator
    \else                      
    \dfrac{#1}{#2\hfill}   % otherwise put \hfill in the denominator
    \fi
}

Using it does not help:

I don't understand why it doesn't work as it seems that the problems are basically the same. Can you help me understand?

Answer 1

Here is another solution, which consists in adding a phantom prescript to the norm. As the fraction line might a little wide, acorrection might be necessary (more typing… but part of the correction can be incorporated to the norm command. Also, as I don't have the physics package installed, I defined the norm command with the help of mathtools:

\documentclass{article}

\usepackage{mathtools}
%\usepackage{physics}
\DeclarePairedDelimiter\norm{\lVert}{\rVert}

\newcommand{\mycommand}[1]{\ensuremath{{#1}_{p}}}

\renewcommand{\vec}[1]{\ensuremath{\mathbf{#1}}}

\newcommand{\normv}[2][p]{%
    \ensuremath{{}_{\phantom{#1}}\norm{\vec{#2}}_{#1}}%
}

\newcommand{\normw}[2][p]{%
    \ensuremath{{}_{\!\phantom{#1}}\norm{\vec{#2}}_{#1}\!}%
}

\begin{document}
    \begin{equation*}
        0 = \frac{x}{\mycommand{x}}
    \end{equation*}

    \begin{equation*}
        \vec{x} = \frac{\vec{x}}{\normv{x}}
    \end{equation*}

    \begin{equation*}
        \vec{x} = \frac{\!\vec{x}\!}{\normw{x}}
    \end{equation*}

\end{document}