Distinguish between escape sequences that produce no output

whatsoever (for example \fR) and escape sequences that produce
invisible zero-width output (for example \&).  No, i'm not joking,
groff does make that distinction, and it has consequences in some
situations, for example for vertical spacing in no-fill mode.
Heirloom and Plan 9 behaviour is subtly different, but in case of
doubt, we want to follow groff.

While this fixes the behaviour for the majority of escape sequences,
in particular for those most likely to occur in practice, it is not
perfect yet because some of the more exotic ESCAPE_IGNORE sequences
are actually of the "no output whatsoever" type but treated
as "invisible zero-width" for now.  With the new ASCII_NBRZW mechanism
in place, switching them over one by one when the need arises will
no longer be very difficult.

1 files changed, 9 insertions, 1 deletions

diff --git a/regress/mdoc/Bd/blank.out_markdown b/regress/mdoc/Bd/blank.out_markdown
index 215d045c..8b97bd6f 100644
--- a/regress/mdoc/Bd/blank.out_markdown
+++ b/regress/mdoc/Bd/blank.out_markdown
@@ -14,6 +14,14 @@ BD-BLANK(1) - General Commands Manual
 	
 	line containing space tab space:
 	
+	line containing a zero-width space:
+	
+	line containing an invalid numbered character escape:
+	<?>
+	line containing an invalid named character escape:
+	<?>
+	line containing a font escape:
+	
 	line starting with a blank character:
 	 x
 	line starting with two blank characters:
@@ -30,4 +38,4 @@ An empty one-line literal display:
 
 end of test document
 
-OpenBSD - July 4, 2017
+OpenBSD - August 15, 2022