Limitations

Highlighting with ucharclasses

The ucharclasses package allows you to define font transitions based on Unicode character classes. However, when using it in combination with the soul package for highlighting, there can be compatibility issues. For example, the following code may not work as expected:

\documentclass{article}
\usepackage{soul}
\usepackage{fontspec}
\usepackage[Latin]{ucharclasses}

\setmainfont{Latin Modern Roman}
\setDefaultTransitions{\rmfamily}{\rmfamily}
\enableTransitionRules
\AtBeginDocument{\enableTransitionRules}

\begin{document}
\hl{Foobar}
\end{document}

TeXSmith is meant to be compatible with XeLaTeX and LuaLaTeX, but due to the way ucharclasses handles font transitions, it may interfere with the highlighting functionality provided by soul. When TeXSmith detects XeLaTeX it now disables soul and falls back to a plain yellow \hl{...} (no box). With LuaLaTeX it loads lua-ul instead of soul, keeping underlines without tripping over ucharclasses. Output therefore differs slightly between XeLaTeX and LuaLaTeX, but both builds succeed.