Use 8 for underscore colour defaults instead of 0 which is less

confusing, and fix writing tge default colour. GitHub issue 3627.

grid.c

@@ -37,20 +37,20 @@
 
 /* Default grid cell data. */
 const struct grid_cell grid_default_cell = {
-	{ { ' ' }, 0, 1, 1 }, 0, 0, 8, 8, 0, 0
+	{ { ' ' }, 0, 1, 1 }, 0, 0, 8, 8, 8, 0
 };
 
 /*
  * Padding grid cell data. Padding cells are the only zero width cell that
  * appears in the grid - because of this, they are always extended cells.
  */
 static const struct grid_cell grid_padding_cell = {
-	{ { '!' }, 0, 0, 0 }, 0, GRID_FLAG_PADDING, 8, 8, 0, 0
+	{ { '!' }, 0, 0, 0 }, 0, GRID_FLAG_PADDING, 8, 8, 8, 0
 };
 
 /* Cleared grid cell data. */
 static const struct grid_cell grid_cleared_cell = {
-	{ { ' ' }, 0, 1, 1 }, 0, GRID_FLAG_CLEARED, 8, 8, 0, 0
+	{ { ' ' }, 0, 1, 1 }, 0, GRID_FLAG_CLEARED, 8, 8, 8, 0
 };
 static const struct grid_cell_entry grid_cleared_entry = {
 	{ .data = { 0, 8, 8, ' ' } }, GRID_FLAG_CLEARED
@@ -528,7 +528,7 @@ grid_get_cell1(struct grid_line *gl, u_int px, struct grid_cell *gc)
 	gc->bg = gce->data.bg;
 	if (gce->flags & GRID_FLAG_BG256)
 		gc->bg |= COLOUR_FLAG_256;
-	gc->us = 0;
+	gc->us = 8;
 	utf8_set(&gc->data, gce->data.data);
 	gc->link = 0;
 }
@@ -956,7 +956,7 @@ grid_string_cells_code(const struct grid_cell *lastgc,
 	for (i = 0; i < nitems(attrs); i++) {
 		if (((~attr & attrs[i].mask) &&
 		    (lastattr & attrs[i].mask)) ||
-		    (lastgc->us != 0 && gc->us == 0)) {
+		    (lastgc->us != 8 && gc->us == 8)) {
 			s[n++] = 0;
 			lastattr &= GRID_ATTR_CHARSET;
 			break;

input.c

@@ -2186,7 +2186,7 @@ input_csi_dispatch_sgr(struct input_ctx *ictx)
 			gc->attr &= ~GRID_ATTR_OVERLINE;
 			break;
 		case 59:
-			gc->us = 0;
+			gc->us = 8;
 			break;
 		case 90:
 		case 91:

tty.c

@@ -2815,9 +2815,10 @@ tty_check_us(__unused struct tty *tty, struct colour_palette *palette,
 	}
 
 	/* Underscore colour is set as RGB so convert. */
-	gc->us = colour_force_rgb (gc->us);
-	if (gc->us == -1)
+	if ((c = colour_force_rgb (gc->us)) == -1)
 		gc->us = 8;
+	else
+		gc->us = c;
 }
 
 static void
@@ -2892,7 +2893,7 @@ tty_colours_us(struct tty *tty, const struct grid_cell *gc)
 	u_char			 r, g, b;
 
 	/* Clear underline colour. */
-	if (gc->us == 0) {
+	if (COLOUR_DEFAULT(gc->us)) {
 		tty_putcode(tty, TTYC_OL);
 		goto save;
 	}