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.

1 files changed, 15 insertions, 19 deletions

diff --git a/term.c b/term.c
index b06fe569..2795cc5c 100644
--- a/term.c
+++ b/term.c
@@ -268,22 +268,11 @@ 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;
@@ -317,12 +306,19 @@ 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;