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[bug #57448] groff will not break a line at a hard hyphen following some letter combinations

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  [bug #57448] groff will not break a line at a hard hyphen following some letter combinations Dave <INVALID.NOREPLY@gnu.org> - 2019-12-20 05:36 -0500

#1581 — [bug #57448] groff will not break a line at a hard hyphen following some letter combinations

Follow-up Comment #3, bug #57448 (project groff):

But you are correct that kerning appears to be what triggers the bug.

In the default font (TR), only four uppercase letters have kerning between
them and a following hyphen:

$ sed -n '/^kernpairs/,/^$/p' /usr/share/groff/current/font/devps/TR | grep
'^[[:upper:]] - '
T - -92
V - -100
W - -65
Y - -111

(Side note: these four letters' glyphs are basically symmetrical, so it's
unclear why this file lacks the same four combinations in reverse order; there
are no kern pairs beginning with a hyphen.)

Now look at all kern pairs beginning with any uppercase letter and ending with
one of the above four:

$ sed -n '/^kernpairs/,/^$/p' /usr/share/groff/current/font/devps/TR | grep
'^[[:upper:]] [TVWY]'

This generates my list of the 19 offending combinations.  Thus a more succinct
bug description would be: groff fails to break a line at a hyphen following
two uppercase letters when the font file specifies kerning both between those
letters and between the second letter and the hyphen.

Curiously, the place where the line break should occur is never kerned: the
break should always be after the hyphen, and no kern pairs have a hyphen as
the first element.  It is kerning that occurs before (what should be) a
breakpoint that somehow inhibits breaking at that point.

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