label lost in overfull box in breakable tcolorbox

[proteusGIT]

Hi,

the label lab1 should not be lost due to the overfull box

\documentclass{article}

\usepackage{tcolorbox}
\tcbuselibrary{breakable}
\begin{document}

\begin{tcolorbox}[breakable]
\label{lab1}%
\rule{2cm}{19cm}
\end{tcolorbox}
lab1 \pageref{lab1} foo

\end{document}

[T-F-S]

I can confirm the problem, but currently I do not see a solution by changing the break algorithm. I did a lot of experiments, but my changes for the algorithm produced new and more severe problems for other examples.

Fortunately, this problem can be easily circumvented. As far as I have seen, it can only happen at the very begin of a tcolorbox. Therefore, the label is a label for the tcolorbox itself and should preferably be put inside the options for the tcolorbox (see lab2 in the code below).

Another way of circumvention is to add an \mbox{} before the label, see lab3 below.

\documentclass{article}

\usepackage{tcolorbox}
\tcbuselibrary{breakable}

\begin{document}

\begin{tcolorbox}[breakable]
\label{lab1}
\rule{2cm}{19cm}
\end{tcolorbox}
Problem: lab1 \pageref{lab1} foo

\clearpage

\begin{tcolorbox}[breakable,label=lab2]
\rule{2cm}{19cm}
\end{tcolorbox}
No problem: lab2 \pageref{lab2} foo

\clearpage

\begin{tcolorbox}[breakable]
\mbox{}\label{lab3}
\rule{2cm}{19cm}
\end{tcolorbox}
No problem: lab3 \pageref{lab3} foo

\end{document}

So, currently, I hesitate to change the break algorithm, because changes typically produce a lot of new problems for existing documents and I was not able to find a bullet-proof solution. Also, I hope that the easy ways of circumvention moderate the weight of the problem.

[proteusGIT]

This solves the problem of course.
Also, I think, people will not overlook missing references.
However, maybe tcolorbox could detect the problem an typeout some hint.

[T-F-S]

For the next version, I added a new warning Discard zero height first box part due to break problems (possible loss of zero height content) for the situation.

[T-F-S]

A warning Discard zero height first box part due to break problems (possible loss of zero height content) is implemented for tcolorbox 4.13 (2018/03/22).

[T-F-S]

In the same situation, an \index can get lost as just happened with an document of mine.

The new warning also helped here :-)

[TinchoteSK]

Thanks a lot for the analysis and the workaround. I had lots of boxes giving me that problem!

[muzimuzhi]

Another way of circumvention is to add an \mbox{} before the label, see lab3 below.

Or \leavevmode. (Sorry for my bothering.)

[TinchoteSK]

Thanks :) The mbox hack worked for me, so I'm not touching it again :D