[Tex/LaTex] Color highlighting with soul package does not work well with ligatures

Question

The picture below says it all. Color highlighting with the soul package prevents a correct rendering of the ff ligature. Is there a way to fix this?

\documentclass{article}
\usepackage{color, soul}
\begin{document}
off -- offer \hl{off -- offer}
\end{document}

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EDIT

cfr's answer below demonstrates that the problem disappears if the words are boxed with \mbox. This will, however, break hyphenation and line-breaking, features that I would like to retain.

Answer 1

Boxing the text to be highlighted avoids the problem:

\documentclass{article}
\usepackage{xcolor, soul}
\begin{document}
  off -- offer \hl{\mbox{off -- offer}}
\end{document}