下部奇怪的行为

Question

如果箭头小于 .7em 或 .3cm，为什么不放在等号下面？

\documentclass{article}
\usepackage{mathtools}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}
\newcommand{\bigswarrow}[1]{\tikz{\draw[->](#1,3ex)--(0,0);}}
\begin{document}
\foreach \wid in {.1,.2,...,1}
{\bigswarrow{\wid em}}

\foreach \wid in {.1,.2,...,1}
{$\underset{\bigswarrow{\wid em}}{=}$}

\foreach \wid in {.1,.2,...,1}
{\bigswarrow{\wid}}

\foreach \wid in {.1,.2,...,1}
{$\underset{\bigswarrow{\wid}}{=}$}
\end{document}

Answer 1

我相信添加后baseline就能修复该问题。

\documentclass{article}
\usepackage{mathtools}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}
\newcommand{\bigswarrow}[1]{\tikz[baseline]{\draw[->](#1,3ex)--(0,0);}}
\begin{document}
\foreach \wid in {.1,.2,...,1}
{\bigswarrow{\wid em}}

\foreach \wid in {.1,.2,...,1}
{$\underset{\bigswarrow{\wid em}}{=}$}

\foreach \wid in {.1,.2,...,1}
{\bigswarrow{\wid}}

\foreach \wid in {.1,.2,...,1}
{$\underset{\bigswarrow{\wid}}{=}$}
\end{document}

Answer 2

不知道为什么，但这里有一个解决办法。我把下划箭头放在\hbox{...}

\documentclass{article}
\usepackage{mathtools}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}
\newcommand{\bigswarrow}[1]{\tikz{\draw[->](#1,3ex)--(0,0);}}
\begin{document}
\foreach \wid in {.1,.2,...,1}
{\bigswarrow{\wid em}}

\foreach \wid in {.1,.2,...,1}
{$\underset{\hbox{\bigswarrow{\wid em}}}{=}$}

\foreach \wid in {.1,.2,...,1}
{\bigswarrow{\wid}}

\foreach \wid in {.1,.2,...,1}
{$\underset{\hbox{\bigswarrow{\wid}}}{=}$}
\end{document}

另外，向 marmot 致敬，我可以\bigswarrow通过以下方式强制进入基线stackengine：

\usepackage{stackengine}
\newcommand{\bigswarrow}[1]{\abovebaseline[0pt]{\tikz{\draw[->](#1,3ex)--(0,0);}}}

那么，我就不需要\hboxes 了。