How to stop text from stretching between two rules in \titleformat{\chapter}

Question

I have a specific way of displaying Chapter title headings, by placing the chapter title in between two rules. I use titlesec to achieve this. The issue is, the stretch that LaTeX does over each chapter title page may be different and so the text within the rules is not always exactly centred.

MWE below. Chapter 1 title text is centred, almost perfectly, between the rules (as the \titleformat{\chapter} code was written by looking at this first chapter title). However, for Chapter 2, because the text afterwards is now different, the title text is no longer centred. Please disregard the fonts and sizing. It's the best I could manage to show my issue without just copying my entire actual code. I am sure there are many ways to overcome this. Would an environment do it? But could this be incorporated into titlesec?

\documentclass[10pt,a4paper,twoside]{book}

\newcommand\YUGE{\fontsize{38}{45}\selectfont} % creates an even bigger 'YUGE' font size, at 38pt

\usepackage{titlesec}
\titleformat{\chapter}[display]{\sf\YUGE}{Chapter \thechapter}{-5mm}{\rule{4.5in}{1pt}\vspace{-3mm}\\\rm\huge}[\vspace{-9mm}\rule{4.5in}{1pt}]

\usepackage{lipsum}
\usepackage{parskip}
\setlength{\parskip}{1\baselineskip plus2pt minus2pt}

\begin{document}

\chapter{Example Chapter Title}

\lipsum[1-2]

\chapter{Example Chapter Title}

\lipsum

\end{document}

Answer 1

UPDATED

Unfortunately your MWE does not show the stretching problem that occurs in your actual work.

To make the chapter title unaffected by changes in the parskip, use the package parskip (2021-03-14 v2) to set the parskip value between paragraphs.

For example \usepackage[skip=1.0\baselineskip plus 1pt minus 1pt]{parskip}

The package also patches titlesec to avoid \parskip being added before or after display headings, thus avoiding unexpected vertical spaces. See this answer

The \titleformat might be improved and simplified using \titleline[<rule height>] (unless you wanted shorter lines), avoiding the \\ and preventing the parskips inserted by the rules .

The text after the title should not make any difference to the title format or position.

To compare both pages side by side use \documentclass[10pt,a4paper]{report}

\documentclass[10pt,a4paper,twoside]{book}
    
\newcommand\YUGE{\fontsize{38}{45}\selectfont} % creates an even bigger 'YUGE' font size, at 38pt

\usepackage{titlesec}

%%\titleformat{\chapter}[display]
%{\sf\YUGE}
%{Chapter \thechapter}
%{-5mm}
%{\rule{4.5in}{1pt}\vspace{-3mm}\\\rm\huge}
%[\vspace{-9mm}\rule{4.5in}{1pt}]

\titleformat{\chapter}[display]
{\parskip=0pt\sf\YUGE} % format Chapter nn
{\chaptertitlename\ \thechapter}
{10mm} % space below Chapter nn
{\parskip=0pt\rm\huge\titlerule[1.0pt]\vspace*{4pt}} % format title
[\parskip=0pt\vspace*{3pt}\titleline{\titlerule[1.0pt]}] % after title

%%  \titlespacing{command}{left spacing}{before spacing}{after spacing}
\titlespacing*{\chapter}{0em}{2em}{6em}  % added <<<<<<<

\usepackage[skip=1.0\baselineskip  plus 1pt  minus 1pt]{parskip} % <<<<<<<<<<<< changed 

\usepackage{lipsum}
    
\begin{document}
    
    \chapter{Example Chapter Title}
    
    \lipsum[1-2]
    
    \chapter{Example Chapter Title}
    
    \lipsum
    
\end{document}