The revenge of the vboxes

Question

Yes, they're back. With a vengeance. Those pesky little buggers we call \vboxes which I've fought against previously. (And Bruno, too.)

\parskip=0pt
\def\unquo{\unvbox0\smallskip}
\def\bquo#{\smallskip\dimen255=\prevdepth\setbox0\vbox\bgroup\prevdepth=\dimen255
  \narrower\noindent\strut\aftergroup\unquo\let\next= }

\hsize=20pc
Some text which goes on for a while, in fact, hopefully long enough to
produce a line break.
\bquo{A quote which goes on for a while, so long in fact, that I was hoping to
see a line break somewhere durin this ouote.}
And then some more text.
\bye

No! What is that space? Not the same amount as before the quote I can tell you that much. Sure you'll tell me \smallskipamount is 3pt plus 1pt minus 1pt, but that can't be 2pt. No way.

Halp. Please.

Answer 1

You need to reset the \prevdepth: doing \unvbox0 doesn't do it.

The TeXbook, page 282:

• \unvbox<8 bit number>, … The value of \prevdepth is not affected.

On the other hand, doing \box will affect the \prevdepth (same page), so we solve the issue by detaching the last box and reattaching it with \box.

\parskip=0pt
\def\unquo{\unvbox0 \setbox0=\lastbox\nointerlineskip\box0 \smallskip}
\def\bquo#{\smallskip\dimen255=\prevdepth\setbox0\vbox\bgroup\prevdepth=\dimen255
  \narrower\noindent\strut\aftergroup\unquo\let\next= }

\hsize=20pc

\dimen0=5\baselineskip
\advance\dimen0 2\smallskipamount
\def\ruler{\leavevmode\smash{\vrule height 0pt depth \dimen0}\kern-0.4pt}


\ruler
Some text which goes on for a while, in fact, hopefully long enough to
produce a line break.
\bquo{A quote which goes on for a while, so long in fact, that I was hoping to
see a line break somewhere durin this quote.}
And then some more text.
\bye

I've added a ruler which has the desired length to show that the spacing is correct.

The \unquo macro can also be defined more compactly by

\def\unquo{\unvbox0 \nointerlineskip\lastbox\smallskip}

Of course the same result can be obtained by saying

\long\def\bquo#1{\smallskip{\narrower#1\par}\smallskip}

Answer 2

You'll get the same spacing if you change ouote to guote so it has the same depth as the first.

adding

\tracingoutput1
\tracingonline2
\showboxbreadth20
\showboxdepth3

The difference is the baselinskip values before the rows starting with A. \unvbox doesn't propagate the depth in the way you need.:

This is TeX, Version 3.1415926 (TeX Live 2011/Cygwin) (format=tex 2012.5.4)  12 JUL 2012 18:21
**ssp
(./ssp.tex

Completed box being shipped out [1]
\vbox(667.20255+0.0)x240.0
.\vbox(0.0+0.0)x240.0, glue set 14.0fil
..\glue -22.5
..\hbox(8.5+0.0)x240.0, glue set 240.0fil
...\vbox(8.5+0.0)x0.0
...\glue 0.0 plus 1.0fil
..\glue 0.0 plus 1.0fil minus 1.0fil
.\vbox(643.20255+0.0)x240.0, glue set 567.20255fill
..\glue(\topskip) 3.05556
..\hbox(6.94444+1.94444)x240.0, glue set - 0.26826
...\hbox(0.0+0.0)x20.0
...\tenrm S
...\tenrm o
...\tenrm m
...\tenrm e
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm t
...\tenrm e
...\tenrm x
...\tenrm t
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm w
...\tenrm h
...\tenrm i
...\tenrm c
...\kern-0.27779
...\tenrm h
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm g
...\tenrm o
...etc.
..\penalty 400
..\glue(\baselineskip) 3.11111
..\hbox(6.94444+1.94444)x240.0, glue set 56.61072fil
...\tenrm f
...\tenrm u
...\tenrm l
...\tenrm l
...\tenrm y
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm l
...\tenrm o
...\tenrm n
...\tenrm g
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm e
...\tenrm n
...\tenrm o
...\tenrm u
...\tenrm g
...\tenrm h
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm t
...\tenrm o
...etc.
..\glue 3.0 plus 1.0 minus 1.0
..\glue(\baselineskip) 1.55556
..\hbox(8.5+3.5)x240.0, glue set 0.90999
...\glue(\leftskip) 20.0
...\rule(8.5+3.5)x0.0
...\tenrm A
...\glue 3.33333 plus 1.66498 minus 1.11221
...\tenrm q
...\tenrm u
...\tenrm o
...\tenrm t
...\tenrm e
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm w
...\tenrm h
...\tenrm i
...\tenrm c
...\kern-0.27779
...\tenrm h
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm g
...\tenrm o
...\kern0.27779
...etc.
..\penalty 150
..\glue(\baselineskip) 1.55556
..\hbox(6.94444+1.94444)x240.0, glue set 0.3171
...\glue(\leftskip) 20.0
...\tenrm i
...\tenrm n
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm f
...\tenrm a
...\tenrm c
...\tenrm t
...\tenrm ,
...\glue 3.33333 plus 2.08331 minus 0.88889
...\tenrm t
...\tenrm h
...\tenrm a
...\tenrm t
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm I
...\glue 3.33333 plus 1.66498 minus 1.11221
...\tenrm w
...\kern-0.27779
...\tenrm a
...etc.
..\penalty 150
..\glue(\baselineskip) 3.11111
..\hbox(6.94444+1.94444)x240.0, glue set 76.49979fil
...\glue(\leftskip) 20.0
...\tenrm s
...\tenrm o
...\tenrm m
...\tenrm e
...\tenrm w
...\tenrm h
...\tenrm e
...\tenrm r
...\tenrm e
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm d
...\tenrm u
...\tenrm r
...\tenrm i
...\tenrm n
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm t
...\tenrm h
...\tenrm i
...etc.
..\glue 3.0 plus 1.0 minus 1.0
..\glue(\parskip) 0.0
..\glue(\baselineskip) 3.11111
..\hbox(6.94444+0.0)x240.0, glue set 104.91646fil
...\hbox(0.0+0.0)x20.0
...\tenrm A
...\tenrm n
...\tenrm d
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm t
...\tenrm h
...\tenrm e
...\tenrm n
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm s
...\tenrm o
...\tenrm m
...\tenrm e
...\glue 3.33333 plus 1.66666 minus 1.11111
...\tenrm m
...\tenrm o
...\tenrm r
...\tenrm e
...\glue 3.33333 plus 1.66666 minus 1.11111
...etc.
..\glue 0.0 plus 1.0fill
.\glue(\baselineskip) 17.55556
.\hbox(6.44444+0.0)x240.0, glue set 117.5fil
..\glue 0.0 plus 1.0fil minus 1.0fil
..\tenrm 1
..\glue 0.0 plus 1.0fil minus 1.0fil

 )
Output written on ssp.dvi (1 page, 540 bytes).