Why do my lines not break correctly?

Question

I'm struggling with line breaks in my document and it seems to me that LaTeX is randomly deciding when to break correctly and when not. See the differences in the three blocks below.

\documentclass[11pt, oneside, a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage{fontspec}
\setmainfont[BoldFont=Calibri, ItalicFont=CalibriLightItalic]{Calibri Light}
\usepackage{xcolor}
\usepackage{lipsum}
\usepackage[normalem]{ulem}
\newcommand{\eolyemph}[1]{\textcolor{blue}{\emph{#1}}}
\usepackage[hidelinks]{hyperref}

\begin{document}
\textbf{\underline{This is too long:}}

Code was written with the help of \href{https://tex.stackexchange.com}{\uline{tex.stackexchange.com}} (all hail \emph{StackExchange}). There, many friendly people from the community are helping each other out on all sorts of computer-related problems.

\textbf{\underline{But this is fine:}}

Code was written with the help of \href{https://tex.stackexchange.com}{\uline{tex.stackexchange.com}} (all hail \emph{TexStackExchange}). There, many friendly people from the community are helping each other out on all sorts of computer-related problems.

\textbf{\underline{This works, too:}}

This document was written with the help of \href{https://tex.stackexchange.com}{\uline{tex.stackexchange.com}} (all hail \emph{StackExchange}). There, many friendly people from the community are helping each other out on all sorts of computer-related problems.

\end{document}

Why does it sometimes break fine and sometimes not?

---

Edit: added screenshot

Answer 1

There is nothing strange, actually. If I try \showhyphens{StackExchange} the answer is

Stack-Ex-change

Those are the only points TeX is willing to break a line at. In your first attempt, breaking after x would produce a line where interword spaces are stretched a lot. This doesn't happen in the other two cases. Here's a slightly modified version of your code (I don't own Calibri Light, but adding one millimeter to \textwidth faithfully reproduces your issue) where the effect of breaking the line after x.

\documentclass[11pt, oneside, a4paper]{article}

\usepackage{fontspec}
\usepackage[normalem]{ulem}
\usepackage{hyperref}

\setmainfont{Calibri}

\addtolength{\textwidth}{0.1cm}

\begin{document}
\textbf{\underline{This is overfull:}}

Code was written with the help of 
\href{https://tex.stackexchange.com}{\uline{tex.stackexchange.com}} 
(all hail \emph{StackExchange}). There, many friendly people from the community 
are helping each other out on all sorts of computer-related problems.

\textbf{\underline{This is how it would be:}}

Code was written with the help of 
\href{https://tex.stackexchange.com}{\uline{tex.stackexchange.com}} 
(all hail \emph{StackEx-\linebreak change}). There, many friendly people from the community 
are helping each other out on all sorts of computer-related problems.

\textbf{\underline{But this is fine:}}

Code was written with the help of 
\href{https://tex.stackexchange.com}{\uline{tex.stackexchange.com}} 
(all hail \emph{TexStackExchange}). There, many friendly people from the community 
are helping each other out on all sorts of computer-related problems.

\textbf{\underline{This works, too:}}

This document was written with the help of 
\href{https://tex.stackexchange.com}{\uline{tex.stackexchange.com}} 
(all hail \emph{StackExchange}). There, many friendly people from the community 
are helping each other out on all sorts of computer-related problems.

\end{document}

Instead of producing a very bad line, TeX prefers to output an overfull one, warning you it did, so you can examine the offending paragraph and reword it to make it fit.

However, adding microtype can be very helpful.

\documentclass[11pt, oneside, a4paper]{article}

\usepackage{fontspec}
\usepackage[normalem]{ulem}
\usepackage{microtype}
\usepackage{hyperref}

\setmainfont{Calibri}

\addtolength{\textwidth}{0.1cm}

\begin{document}
\textbf{\underline{This is acceptable:}}

Code was written with the help of 
\href{https://tex.stackexchange.com}{\uline{tex.stackexchange.com}} 
(all hail \emph{StackExchange}). There, many friendly people from the community 
are helping each other out on all sorts of computer-related problems.


\microtypesetup{protrusion=false,expansion=false}

\textbf{\underline{This is how it would be without microtype:}}

Code was written with the help of 
\href{https://tex.stackexchange.com}{\uline{tex.stackexchange.com}} 
(all hail \emph{StackEx-\linebreak change}). There, many friendly people from the community 
are helping each other out on all sorts of computer-related problems.

\end{document}