From dcd30e5b2079d164347b81ed31494c8871cb163b Mon Sep 17 00:00:00 2001
From: schwarze <schwarze@openbsd.org>
Date: Mon, 15 Aug 2022 10:21:01 +0000
Subject: [PATCH] In GNU, Heirloom, and Plan 9 roff, literal tab characters are
 non-breakable in exactly the same way as "\ ".  That is, the preceding word,
 the tab character, and the following word are always kept together on the
 same output line.  If filling is enabled and an output line break is required
 before the end of the following word, the break occurs before the beginning
 of the preceding word.

Make mandoc behave in the same way.

Of course, using literal tab characters in filled text remains a
bad idea, and the "WARNING: tab in filled text" remains unchanged.
---
 usr.bin/mandoc/term.c | 36 ++++++++++++++++--------------------
 1 file changed, 16 insertions(+), 20 deletions(-)

diff --git a/usr.bin/mandoc/term.c b/usr.bin/mandoc/term.c
index f89da26450e..b57296c45ce 100644
--- a/usr.bin/mandoc/term.c
+++ b/usr.bin/mandoc/term.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: term.c,v 1.146 2022/04/27 13:30:19 schwarze Exp $ */
+/* $OpenBSD: term.c,v 1.147 2022/08/15 10:21:01 schwarze Exp $ */
 /*
  * Copyright (c) 2010-2022 Ingo Schwarze <schwarze@openbsd.org>
  * Copyright (c) 2008, 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
@@ -266,22 +266,11 @@ term_fill(struct termp *p, size_t *nbr, size_t *vbr, size_t vtarget)
 			vis -= (*p->width)(p, p->tcol->buf[ic - 1]);
 			continue;
 
-		case '\t':  /* Normal ASCII whitespace. */
 		case ' ':
 		case ASCII_BREAK:  /* Escape \: (breakpoint). */
-			switch (p->tcol->buf[ic]) {
-			case '\t':
-				vn = term_tab_next(vis);
-				break;
-			case ' ':
-				vn = vis + (*p->width)(p, ' ');
-				break;
-			case ASCII_BREAK:
-				vn = vis;
-				break;
-			default:
-				abort();
-			}
+			vn = vis;
+			if (p->tcol->buf[ic] == ' ')
+				vn += (*p->width)(p, ' ');
 			/* Can break at the end of a word. */
 			if (breakline || vn > vtarget)
 				break;
@@ -315,12 +304,19 @@ term_fill(struct termp *p, size_t *nbr, size_t *vbr, size_t vtarget)
 			*vbr = vis;
 			continue;
 
-		case ASCII_NBRSP:  /* Non-breakable space. */
-			p->tcol->buf[ic] = ' ';
-			/* FALLTHROUGH */
-		default:  /* Printable character. */
+		default:
+			switch (p->tcol->buf[ic]) {
+			case '\t':
+				vis = term_tab_next(vis);
+				break;
+			case ASCII_NBRSP:  /* Non-breakable space. */
+				p->tcol->buf[ic] = ' ';
+				/* FALLTHROUGH */
+			default:  /* Printable character. */
+				vis += (*p->width)(p, p->tcol->buf[ic]);
+				break;
+			}
 			graph = 1;
-			vis += (*p->width)(p, p->tcol->buf[ic]);
 			if (vis > vtarget && *nbr > 0)
 				return;
 			continue;
-- 
2.20.1