[bug #60731] undocumented difference in .ce behavior between groff and traditional troff

[Dave]

URL:
  <https://savannah.gnu.org/bugs/?60731>

                 Summary: undocumented difference in .ce behavior between
groff and traditional troff
                 Project: GNU troff
            Submitted by: barx
            Submitted on: Sat 05 Jun 2021 01:35:57 AM CDT
                Category: Core
                Severity: 2 - Minor
              Item Group: Incorrect behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None

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Details:

= Preamble =

It's not clear to me whether this is a behavior bug or a documentation
omission.  That is, I don't know whether groff's authors deliberately made
this .ce behavior different from historical troff and merely forgot to
document it, or whether they intended .ce to work as it historically did.  I'm
filing it as "Incorrect behaviour" because I had to pick one or the other, but
it could as easily be switched to "Documentation."

I'm using Heirloom troff as the standard bearer here for "traditional troff,"
though I realize that's a simplification.  But it's what I have access to, and
it does demonstrate a notable difference.

= Documentation =

The Texinfo manual says .ce centers text "without filling" but is silent on
breaking.  

On the other hand, the description of .ce in section 4.2 of CSTR #54 does not
mention filling, but does (almost) say that no breaking occurs: "If the input
line is too long, it will be left adjusted."

= Behavior =

To see the behavioral difference between groff and Heirloom troff, run the
example code from the .ce section of the latest (as of this writing, commit
16a0fc88) version of the Texinfo manual. 
(http://git.savannah.gnu.org/cgit/groff.git/tree/doc/groff.texi?id=16a0fc88#n7128
links directly to this example.)  This example is designed to show the
difference in filling behavior between .ce and ".ad c", but also shows the
difference in breaking behavior between groff and Heirloom.

Groff (1.23.0.rc1) produces the output (in nroff) the manual says it does,
showing that .ce input is broken but not filled:


This is a small text fragment that shows
            the differences
between the `.ce' and the `.ad c' requests.

This is a small text fragment that shows
 the differences between the `.ce' and
         the `.ad c' requests.



In Heirloom nroff, however, .ce input is neither broken nor filled.


This is a small text fragment that shows the differences
between the `.ce' and the `.ad c' requests.

This is a small text fragment that shows
 the differences between the `.ce' and
         the `.ad c' requests.



The traditional nonbreaking behavior does also mean that the line overruns the
requested line length (as would any text where breaking is explicitly
disabled).

The behavior difference holds for typeset output as well, though the line
length in the example needs to be reduced (from 4i to, say, 2i) to see the
difference.




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