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What does the black rectangle at the end of the line mean? It is overflow?

enter image description here

This writes overflow

\documentclass[12pt,a4paper]{report}
\setlength\textwidth{145mm}
\setlength\textheight{247mm}
\setlength\oddsidemargin{15mm}
\setlength\evensidemargin{15mm}
\setlength\topmargin{0mm}
\setlength\headsep{0mm}
\setlength\headheight{0mm}
\let\openright=\clearpage

\begin{document}

\begin{description}

\item [Epocha (ekvinokcium)]
$T$ udává okamžik v čase. Je několik druhů epoch - referenční epocha (v případě světelných křivek udává, kdy je fáze 0), epocha maximální radiální rychlosti, epocha periastra, epocha primárního minima, apod. Udává se v různých časových jednotkách: 

\begin{itemize}
\item Juliánská data se hodí na analýzu dlouhých časových pozorování. Označují střední sluneční dny, které začínají v poledne světového času SČ, což je lokální čas na poledníku procházejícím observatoří v Greenwichi. Počátek těchto časových jednotek JD0 připadá na střední poledne v Greenwichi 1. ledna 4713 př. n. l.  

\end{itemize}

\end{description}

\end{document}

marked as duplicate by CarLaTeX, Stefan Pinnow, TeXnician, egreg symbols Dec 8 '18 at 21:32

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  • 5
    Usually it indicates an overfull box, but you have to enable this manually. – TeXnician Dec 8 '18 at 15:27
  • 2
    In draft mode (or with other 'debug'-like settings) those boxes indicate overfull boxes, which means in that case that the line protrudes into the margin because LaTeX can't find a good point to break it. – moewe Dec 8 '18 at 15:31
  • 3
    Do you want to enable or disable the black box? I.e. do you have a box at the moment and do you want to get rid of it or have it? – moewe Dec 8 '18 at 16:55
  • 2
    @moewe - I've withdrawn the duplicate-posting note, per your request. Note that the warning the OP is encountering in his/her code has nothing to do with overfull-line slugs, as one might be tempted to assume is the case on the basis of the posting's title. – Mico Dec 8 '18 at 20:50
  • 3
    Related: tex.stackexchange.com/q/138/35864 and tex.stackexchange.com/q/35/35864. Concrete help is always a bit tricky. I can still reproduce an overfull box in your example if I add fontenc and babel (though it is far better than before): microtype might help, sometimes it is also recommended to just rewrite/reformulate the offending sentence so it breaks nicer (cf. tex.stackexchange.com/q/20585/35864, tex.stackexchange.com/q/209229/35864) – moewe Dec 9 '18 at 7:19

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