Removing spacing due to \overbrace \underbrace

Question

I'm currently using the nice hack defining \smalloverbrace for reducing the size of my \overbrace and \underbrace.

%% Commands for more compact over and underbraces
%% \smalloverbrace
%% See: https://tex.stackexchange.com/a/419690/110759
\makeatletter
\def\smalloverbrace#1{\mathop{\vbox{\m@th\ialign{##\crcr\noalign{\kern3\p@}%
  \tiny\downbracefill\crcr\noalign{\kern3\p@\nointerlineskip}%
  $\hfil\displaystyle{#1}\hfil$\crcr}}}\limits}

\def\smallunderbrace#1{\mathop {\vtop {\m@th \ialign {##\crcr $\hfil \displaystyle {#1}\hfil $\crcr \noalign {\kern 3\p@ \nointerlineskip }
\tiny\upbracefill \crcr \noalign {\kern 3\p@ }}}}\limits}
\makeatother

\newcommand{\clap}[1]{\makebox[0pt]{#1}}
%% end code for \smalloverbrace and \smallunderbrace

I'd like to modify it further so that the spacing between math elements ignores the size of the text in the over/underbrace. (I understand you may think this is unwise, but I have a need for it in making lecture slides.)

Here's an example using the current set up

\begin{align*}
\Pr( \text{Lost at } t = 1)  &\approx  \smallunderbrace{(1-s)}_{\shortstack{Effect of \\ Natural selection}} \smalloverbrace{0.368}^{\shortstack{Pr. of loss of\\neutral allele}}
\end{align*}

Here's an example of the desired output (I got the effect I wanted fiddling with \hspace*{}

\begin{align*}
\Pr( \text{Lost at } t = 1)  &\approx \hspace*{-0.25in} \underbrace{(1-s)}_{\small \shortstack{Effect of \\ Natural selection}} \hspace*{-0.4in}\overbrace{0.368}^{\small \shortstack{Pr. of loss of\\neutral allele}}
\end{align*}

Could someone tell me how I can modify the above code to get the desired results?

Answer 1

I'd use "ordinary" \underbrace and \overbrace directives, and I wouldn't bother with inserting line breaks.

\documentclass{article} % or some other suitable document class
\usepackage{mathtools}  % for '\mathclap' macro

\begin{document}
\[
\Pr(\text{Loss at $t = 1$}) 
\approx  
{\underbrace{(1-s)}_{\mathclap{\text{Effect of natural selection}}}}
\times
{\overbrace{0.368}^{\mathclap{\text{Pr.\ of loss of neutral allele}}}}
\]
\end{document}

Addendum: In case readers are curious why I surrounded the \underbrace{...}_{...} and \overbrace{...}^{...} objects with curly braces: Doing so changes their TeX math status to "math-ordinary" which, in turn, improves their spacing relative to other objects such as \approx and \times.