Interference between `ragged2e` and `tcolorbox`: Additional spacing for centered bold text

Question

When centering bold text in a tcolorbox using [halign upper=center], additional horizontal spacing occurs when the package ragged2e is loaded additionally.

I guess this is unintended behavior. – Is it possible to easily circumvent it?

\documentclass{scrartcl}

\usepackage{ragged2e}%    <- comment this for normal functioning
\usepackage{tcolorbox}

\begin{document}

\begin{center}
\bfseries centered bold text 
\end{center}

\begin{tcolorbox}
\bfseries centered bold text
\end{tcolorbox}

\begin{tcolorbox}[halign upper=center]
\bfseries centered bold text
\end{tcolorbox}

\end{document}

Answer 1

ragged2e hooks into the fontselection and sets there \spaceskip to 0pt. This overwrites the settings of tcolorbox. A \spaceskip of 0pt means that the normal interword space is used and in this context this space is stretched.

A local work-around would be to reset the \spaceskip to the value of tcolorbox:

\documentclass{scrartcl}

\usepackage{ragged2e}%    <- comment this for normal functioning
\usepackage{tcolorbox}

\begin{document}

\begin{tcolorbox}[halign upper=center]
\bfseries centered bold text
\end{tcolorbox}

\begin{tcolorbox}[halign upper=center]
\bfseries\spaceskip0.3333em\relax centered bold text
\end{tcolorbox}


\end{document}

A better solution would be probably if ragged2e would do nothing (as the documentation claims) if none of its commands is active:

\makeatletter

\renewcommand{\@raggedtwoe@everyselectfont}{%
  \if@raggedtwoe@spaceskip
    \ifdim\fontdimen\thr@@\font=\z@\relax
      \spaceskip\z@
    \else
      \spaceskip\fontdimen\tw@\font
    \fi
  \else
    %\spaceskip\z@ %removed
  \fi
  }