Crash: Verbatim environments followed by blank line at end of question

[nstanger]

Discovered by Mark George. If the very last thing in a question is some sort of verbatim environment (verbatim, lstlisting, etc.) following by a blank line, then ouexam crashes when attempting to insert the number of marks:

! You can't use `\unskip' in vertical mode.
\endquestion ...nning}{\value {qexpected}}\unskip 
                                                  \nobreak \hfil \penalty 50...
l.64 \end{question}

I’m guessing the verbatim environment is putting TeX into vertical mode (they do tend to screw around with things quite a lot).

Things to note:

• Works fine if there’s no trailing blank line.
• Verbatim environments only; \verb is fine.

Minimal (not) working example:

\documentclass{ouexam}

\examyear{2014}
\department{department}
\papernumber{paper}
\papertitle{title}

\begin{document}
\begin{question}{100}
    bla bla

    \begin{verbatim}
        bla bla bla
    \end{verbatim}

\end{question}
\end{document}

Remove the blank line after \end{verbatim} and it’s fine.

[nstanger]

Relevant things from The TeXbook:

• p. 209 (⇒ 220): \ifvmode test for vertical mode.
• pp. 222–223 (⇒ 233–234): “The operation \unskip is something like \lastbox, except that it applies to glue instead of to boxes. If the last thing on the current list is a glue item (or leaders, as explained below), it is removed. You can’t remove glue from the current page by using \unskip in vertical mode, but you can say \vskip-\lastskip, which has almost the same effect.”
• p. 280 (⇒ 291): definition of \unskip (such as it is).
• pp. 86 & 313 (⇒ 97 & 324): exercise 13.1 talks about ways to exit vertical mode.

[nstanger]

The \vskip-\lastskip approach seems to work fine.

[nstanger]

Ah, an interesting wrinkle: it only fails if there is a blank line following the verbatim environment and there is text before the verbatim. No text or whitespace before the verbatim doesn’t trigger the problem, regardless of following whitespace. The solution is still the same regardless.

[nstanger]

After upgrading to TeX Live 2015, the \vskip-\lastskip solution has stopped working. The document builds fine, but the marks are no longer pulled back to the previous line if there’s space. The breakage with \unskip and verbatims still holds.

[nstanger]

It looks like \lastskip is now always zero for some reason. Switching back to \unskip with a \leavevmode in front of it seems to work OK, except perhaps for too much vertical space after an itemize.