In GNU, Heirloom, and Plan 9 roff, tab positions apply to *input* lines,

not to *output* lines.  In particular, if an input line gets broken in
fill mode and a tab occurs in the second output line, it advances to a
position of at least (width of the first output line) + (width of a
space character even though this is never printed) + (width of the part
of the second output line that precedes the tab).

Implement the same logic in mandoc.

Again, do not use tabs in filled text: they have surprising effects,
including this one.

3 files changed, 15 insertions, 10 deletions

diff --git a/usr.bin/mandoc/man_term.c b/usr.bin/mandoc/man_term.c
index 2152cdea691..2433abcdbae 100644
--- a/usr.bin/mandoc/man_term.c
+++ b/usr.bin/mandoc/man_term.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: man_term.c,v 1.190 2022/04/27 17:04:15 schwarze Exp $ */
+/* $OpenBSD: man_term.c,v 1.191 2022/08/15 13:01:40 schwarze Exp $ */
 /*
  * Copyright (c) 2010-2015,2017-2020,2022 Ingo Schwarze <schwarze@openbsd.org>
  * Copyright (c) 2008-2012 Kristaps Dzonsons <kristaps@bsd.lv>
@@ -973,6 +973,7 @@ out:
 	    ! (p->flags & (TERMP_NOBREAK | TERMP_NONEWLINE)) &&
 	    (n->next == NULL || n->next->flags & NODE_LINE)) {
 		p->flags |= TERMP_BRNEVER | TERMP_NOSPACE;
+		p->tcol->taboff = 0;
 		if (n->string != NULL && *n->string != '\0')
 			term_flushln(p);
 		else
diff --git a/usr.bin/mandoc/term.c b/usr.bin/mandoc/term.c
index b57296c45ce..fe098f76c0b 100644
--- a/usr.bin/mandoc/term.c
+++ b/usr.bin/mandoc/term.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: term.c,v 1.147 2022/08/15 10:21:01 schwarze Exp $ */
+/* $OpenBSD: term.c,v 1.148 2022/08/15 13:01:40 schwarze Exp $ */
 /*
  * Copyright (c) 2010-2022 Ingo Schwarze <schwarze@openbsd.org>
  * Copyright (c) 2008, 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
@@ -155,6 +155,7 @@ term_flushln(struct termp *p)
 		/* Finally, print the field content. */
 
 		term_field(p, vbl, nbr);
+		p->tcol->taboff += vbr + (*p->width)(p, ' ');
 
 		/*
 		 * If there is no text left in the field, exit the loop.
@@ -307,7 +308,9 @@ term_fill(struct termp *p, size_t *nbr, size_t *vbr, size_t vtarget)
 		default:
 			switch (p->tcol->buf[ic]) {
 			case '\t':
+				vis += p->tcol->taboff;
 				vis = term_tab_next(vis);
+				vis -= p->tcol->taboff;
 				break;
 			case ASCII_NBRSP:  /* Non-breakable space. */
 				p->tcol->buf[ic] = ' ';
@@ -346,8 +349,8 @@ term_field(struct termp *p, size_t vbl, size_t nbr)
 {
 	size_t	 ic;	/* Character position in the input buffer. */
 	size_t	 vis;	/* Visual position of the current character. */
+	size_t	 vt;	/* Visual position including tab offset. */
 	size_t	 dv;	/* Visual width of the current character. */
-	size_t	 vn;	/* Visual position of the next character. */
 
 	vis = 0;
 	for (ic = p->tcol->col; ic < nbr; ic++) {
@@ -362,13 +365,13 @@ term_field(struct termp *p, size_t vbl, size_t nbr)
 		case ASCII_BREAK:
 			continue;
 		case '\t':
-			vn = term_tab_next(vis);
-			vbl += vn - vis;
-			vis = vn;
-			continue;
 		case ' ':
 		case ASCII_NBRSP:
-			dv = (*p->width)(p, ' ');
+			if (p->tcol->buf[ic] == '\t') {
+				vt = p->tcol->taboff + vis;
+				dv = term_tab_next(vt) - vt;
+			} else
+				dv = (*p->width)(p, ' ');
 			vbl += dv;
 			vis += dv;
 			continue;
@@ -430,7 +433,7 @@ endline(struct termp *p)
 void
 term_newln(struct termp *p)
 {
-
+	p->tcol->taboff = 0;
 	p->flags |= TERMP_NOSPACE;
 	if (p->tcol->lastcol || p->viscol)
 		term_flushln(p);
diff --git a/usr.bin/mandoc/term.h b/usr.bin/mandoc/term.h
index deba9352979..afdc5591f57 100644
--- a/usr.bin/mandoc/term.h
+++ b/usr.bin/mandoc/term.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: term.h,v 1.76 2021/10/04 18:56:24 schwarze Exp $ */
+/* $OpenBSD: term.h,v 1.77 2022/08/15 13:01:40 schwarze Exp $ */
 /*
  * Copyright (c) 2008, 2009, 2010, 2011 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2011-2015, 2017, 2019 Ingo Schwarze <schwarze@openbsd.org>
@@ -56,6 +56,7 @@ struct	termp_col {
 	size_t		  col;		/* Byte in buf to be written. */
 	size_t		  rmargin;	/* Current right margin. */
 	size_t		  offset;	/* Current left margin. */
+	size_t		  taboff;	/* Offset for literal tabs. */
 };
 
 struct	termp {