Tcolorboxes in itemize: additional vertical spacing in first item

Question

Consider following MWE:

\documentclass{article}
\usepackage[most]{tcolorbox}
\definecolor{block-gray}{gray}{0.95}
\newtcolorbox{zitat}[2][]{%
    breakable, % IMPORTANT
    colback=block-gray,
    boxrule=0pt,
    arc=0pt,
    boxsep=0pt,
    enhanced jigsaw,
    borderline west={4pt}{0pt}{gray},
    title={#2\par},
    colbacktitle={block-gray},
    coltitle={black},
    fonttitle={\large\bfseries},
    attach title to upper={},
    box align=top,
    #1,
}
\begin{document}
\section*{Quotes} % commenting removes difference
\begin{itemize}
  \item \begin{zitat}{Quote 1}
    Space: the final frontier.
  \end{zitat}
  \item \begin{zitat}{Quote 2}
    A long time ago in a galaxy far, far away\ldots
  \end{zitat}
  \item \begin{zitat}{Quote 2}
    A long time ago in a galaxy far, far away\ldots
  \end{zitat}
\end{itemize}
\end{document}

which produces following output

As you can see there is more vertical space before tcolorbox in the first item compared to the last two ones.

This disappears when I remove the section command.

What can I do to fix this without manually adjusting it?

(TexLive 2016 on OSX El Capitan, tcolorbox 2016/11/18 version 3.96)

UPDATE:

It appears that the option breakable is responsible. There is a related option vfill before first:

/tcb/vfill before first=true|false Inserts a \vfill at the begin of the first partial box to move this partial box to the end of the current page. This may be used as an alternative to /tcb/height fixed for=first to get justified columns or pages. The \vfill is not inserted, if the box gets not actually broken.

However, removing breakable and inserting a \vfill does not reproduce this issue. I guess the author of tcolorbox probably knows what is going on.

I report this to https://github.com/T-F-S/tcolorbox/issues

Answer 1

tcolorbox takes special care to insert the right spacing after a heading. This is especially tricky, if the tcolorbox should be breakable, because breakable boxes always start a new paragraph. Putting this combination into a list environment like itemize seems to be too much for the algorithm.

Currently, I would'nt know how to handle this automatically without risking to break other mechanisms.

But, there are (at least) two ways to avoid the situation.

Way 1:

Add a \@nobreakfalse after your section heading. This will switch off the special treatment for headings which has a bad effect here.

\documentclass{article}
\usepackage[most]{tcolorbox}
\definecolor{block-gray}{gray}{0.95}
\newtcolorbox{zitat}[2][]{%
    breakable, % IMPORTANT
    colback=block-gray,
    boxrule=0pt,
    arc=0pt,
    boxsep=0pt,
    enhanced jigsaw,
    borderline west={4pt}{0pt}{gray},
    title={#2\par},
    colbacktitle={block-gray},
    coltitle={black},
    fonttitle={\large\bfseries},
    attach title to upper={},
    box align=top,
    #1,
}
\begin{document}
\section*{Quotes} 
\makeatletter\@nobreakfalse\makeatother%
\begin{itemize}
  \item \begin{zitat}{Quote 1}
    Space: the final frontier.
  \end{zitat}
  \item \begin{zitat}{Quote 2}
    A long time ago in a galaxy far, far away\ldots
  \end{zitat}
  \item \begin{zitat}{Quote 2}
    A long time ago in a galaxy far, far away\ldots
  \end{zitat}
\end{itemize}
\end{document}

Way 2:

Add an ignore nobreak option to the first of your zitat environments after the heading. Alternatively, this could be added to the general option list of your environment.

\documentclass{article}
\usepackage[most]{tcolorbox}
\definecolor{block-gray}{gray}{0.95}
\newtcolorbox{zitat}[2][]{%
    breakable, % IMPORTANT
    colback=block-gray,
    boxrule=0pt,
    arc=0pt,
    boxsep=0pt,
    enhanced jigsaw,
    borderline west={4pt}{0pt}{gray},
    title={#2\par},
    colbacktitle={block-gray},
    coltitle={black},
    fonttitle={\large\bfseries},
    attach title to upper={},
    box align=top,
    %  ignore nobreak,  %<---- could also be added here
    #1,
}
\begin{document}
\section*{Quotes} 
\begin{itemize}
  \item \begin{zitat}[ignore nobreak]{Quote 1}      
    Space: the final frontier.
  \end{zitat}
  \item \begin{zitat}{Quote 2}
    A long time ago in a galaxy far, far away\ldots
  \end{zitat}
  \item \begin{zitat}{Quote 2}
    A long time ago in a galaxy far, far away\ldots
  \end{zitat}
\end{itemize}
\end{document}

Both ways should give the desired result.