hyphenation: what special is in word "implementation"

Question

What special is in the word "implementation" that it is not possible automatic hyphenate in narrower environments as are minipage, TikZ nodes etc, even if in preamble is added hyphenation pattern for it?

Interestingly, hyphenation works, if in the begins of those environments is added \hspace{0pt}! Examples:

\documentclass{article}
\begin{document}

\textbullet doesn't work:

    \smallskip
\fbox{\begin{minipage}{4em}
        implementation
      \end{minipage}
      }

\medskip
\textbullet\ with use of \verb+\hspace*{0pt}+ works:

\fbox{\begin{minipage}{4em}\hspace*{0pt} % <---
        implementation
      \end{minipage}
      }
\end{document}

And example at use TikZ nodes:

    \documentclass{article}
    \usepackage{tikz}
    \begin{document}

    \textbullet\ doesn't work:

    \smallskip
    \tikz\node[draw, text width=4em, align=center] {implementation};

     \textbullet\ with use of \verb+\hspace*{0pt}+ works:

    \tikz\node[draw, text width=4em, align=center,
               execute at begin node=\hspace*{0pt}] % <---
               {implementation};
    \end{document}

I wonder, what is in the word implementation. For example, with similar long word hyphenation this problem not occur.

Answer 1

There is nothing special about implementation. TeX simply does not hyphenate the first word of a paragraph. If you need this hyphenated you have to have glue before it such as \hspace{0pt}. This (mis)-feature is shared with tex variants such as ptex and xetex, however luatex does include the first word in its hyphenation pass, and so would hyphenate your example without the extra space.

Answer 2

Hyphenations are based on line breaks to compose a paragraph. The hyphenation algorithm is probably not used at first.

The TeXbook, chapter 6: running TeX

[...] a tolerance of 10 000 will never produce an overfull box, unless something strange occurs like an unhyphenatable word that is wider than the column itself.

The following example from the TeXbook shows an overfull box although the log file shows that the word Drofnats can be hyphenated: Drof-.

\tracingparagraphs=1
\tolerance=10000
\hsize=.75in
\hrule
\vskip 1in
\centerline{\bf A SHORT STORY}
\vskip 6pt
\centerline{\sl    by A. U. Thor} % !`?`?! (modified)
\vskip .5cm
Once upon a time, in a distant
  galaxy called \"O\"o\c c,
there lived a computer
named R.~J. Drofnats.

Mr.~Drofnats---or ``R. J.,'' as
he preferred to be called---% error has been fixed!
was happiest when he was at work
typesetting beautiful documents.
\vskip 1in
\hrule
\vfill\eject
\end

By forcing the hyphenation, we obtain a better rendering.

hfuzz=10pt  %->line 3
Mr~Drof\-\penalty10000nats %->line 16