[Tex/LaTex] \newline not working with longtable

Question

This table comes from SAS output. The \newline is not working correctly. E.g. "Percent trimmed in tail" ought to be

Percent
Trimmed
In tail

but it is showing up as

Percent Trimmed In Tail

Here is a minimal example

\documentclass{article}
\usepackage{natbib}
\usepackage[english]{babel}
\usepackage[latin1]{inputenc}
\usepackage{longtable}

\usepackage{array}
\usepackage{listings}
\usepackage{multicol}
\usepackage{url}
\lstloadlanguages{SAS} \lstset{language = SAS}

\begin{document}
 \begin{longtable}{|r|r|r|r|r|r|r|r|r|}\hline
   \multicolumn{9}{|c|}{Trimmed Means}\\\hline
   Percent {\newline} Trimmed \newline in Tail &    Number \newline Trimmed \newline in Tail &    Trimmed \newline Mean &    Std Error \newline Trimmed \newline Mean &    \multicolumn{2}{|c|}{95\%~Confidence~Limits} &    DF &    t for H0: \newline Mu0=0.00 &    Pr~>~|t|\\\hline
\endhead
   10.27 &    27 &    463.8867 &    25.08152 &    414.4401 &    513.3332 &    208 &    18.49516 &    <.0001\\\hline
\end{longtable}
\end{document}

Answer 1

Now you can write multiline cells without fixing the column widths with the new LaTeX3 package tabularray:

\documentclass{article}
\usepackage[a4paper,left=2cm,right=2cm]{geometry}

\usepackage{tabularray}

\begin{document}

\begin{longtblr}[
  caption = {Long Caption},
  label = {tblr:test},
]{
  colspec={|r|r|r|r|r|r|r|r|r|}, rowhead = 2, row{2}={m},
  cell{1}{1} = {c=9}{c}, cell{2}{5} = {c=2}{c},
}
\hline
 Trimmed Means \\
\hline
 {Percent \\ Trimmed \\ in Tail} & {Number \\ Trimmed \\ in Tail} & {Trimmed \\ Mean} &
 {Std Error \\ Trimmed \\ Mean}  & {$95\%$ \\ Confidence Limits} & & DF &
 {$t$ for $H_0$: \\ $\mu_0=0.00$} & $\mathrm{Pr}>|t|$ \\
\hline
 $10.27$ & $27$ & $463.8867$ & $25.08152$ & $414.4401$ & $513.3332$ & $208$ & $18.49516$ & $<.0001$ \\
\hline
\end{longtblr}

\end{document}