Why does unboxing lose margin kerning in pdfTeX?

Question

Consider the following code:

\showboxdepth=20
\showboxbreadth=200
\hsize=5.33333in
\pdfoutput=0

\pdfprotrudechars=1 
\input protcode.tex
\setprotcode\tenrm


\vbox{\noindent  Test. Test. Test. Test. Test. Test. Test. Test.
Test. Test. Test. Test. Test. Test. Test. Test. Test. Test. Test. Test.
Test. Test. Test. Test. Test. Test. Test. Test. Test. Test.\par
\global\setbox3=\lastbox }
\showbox3 \setbox3=\hbox{\unhbox3 }%
\showbox3

\bye

Now run it with pdftex. The log file shows that the left and right margin \kerns disappear in reboxing. Why? Can it be prevented? Here are excerpts:

> \box3=
\hbox(6.83331+0.0)x385.43962, glue set 3.85056fil
.\kern-0.36 (left margin)
.\tenrm T
.\kern-0.83334
.\tenrm e
.\tenrm s
.\tenrm t
.\tenrm .
.\glue 4.44444 plus 4.99997 minus 0.37036
[cut]
.\tenrm T
.\kern-0.83334
.\tenrm e
.\tenrm s
.\tenrm t
.\tenrm .
.\penalty 10000
.\kern-1.94 (right margin)
.\glue(\parfillskip) 0.0 plus 1.0fil
.\glue(\rightskip) 0.0


! OK.
l.15 \showbox3 
               \setbox3=\hbox{\unhbox3 }%
? 
> \box3=
\hbox(6.83331+0.0)x383.88907
.\tenrm T
.\kern-0.83334
.\tenrm e
.\tenrm s
.\tenrm t
.\tenrm .
[cut]
.\tenrm T
.\kern-0.83334
.\tenrm e
.\tenrm s
.\tenrm t
.\tenrm .
.\penalty 10000
.\glue(\parfillskip) 0.0 plus 1.0fil
.\glue(\rightskip) 0.0

! OK.
l.16 \showbox3

This won't happen with ordinary \kerns, such as if we \setbox3=\hbox{\kern-1pt Test.\kern1pt}.

Answer 1

This is indeed, as @egreg has said in the comments, by design (margin kerning is lost after \unhboxing or \unhcopying). But there are two commands that come as remedy: \leftmarginkern and \rightmarginkern allow access to the the respective kerns of an hbox.

So to adapt your sample code, the following will reinsert margin kerning:

\showboxdepth=20
\showboxbreadth=200
\hsize=5.33333in
%\pdfoutput=0

\pdfprotrudechars=1
\input protcode.tex
\setprotcode\tenrm

\setbox0=\vbox{\noindent  Test. Test. Test. Test. Test. Test. Test. Test.
        Test. Test. Test. Test. Test. Test. Test. Test. Test. Test. Test. Test.
        Test. Test. Test. Test. Test. Test. Test. Test. Test. Test.\par}
\showbox0 

\newbox\splitbox
\setbox3=\vbox{%
  \loop\ifvbox0
    \vbadness=10000
    \splittopskip=0pt
    \setbox\splitbox\vsplit0 to\baselineskip
    \unvbox\splitbox\setbox\splitbox=\lastbox
    \hbox{\kern\leftmarginkern\splitbox
          \expandafter\unhbox\expandafter\splitbox\expandafter
          \kern\rightmarginkern\splitbox}%
  \repeat}

\showbox3
\unvbox3
\bye

(Mind you, there is bug with \rightmarginkern and stretchy \parfillskip as reported here.)