Add pane-border-lines value to use spaces for pane borders, from lukas

dot 23022005 at gmail dot com in GitHub issue 4587.

options-table.c

@@ -77,7 +77,7 @@ static const char *options_table_pane_border_indicators_list[] = {
 	"off", "colour", "arrows", "both", NULL
 };
 static const char *options_table_pane_border_lines_list[] = {
-	"single", "double", "heavy", "simple", "number", NULL
+	"single", "double", "heavy", "simple", "number", "spaces", NULL
 };
 static const char *options_table_popup_border_lines_list[] = {
 	"single", "double", "heavy", "simple", "rounded", "padded", "none", NULL

screen-redraw.c

@@ -87,6 +87,10 @@ screen_redraw_border_set(struct window *w, struct window_pane *wp,
 		gc->attr &= ~GRID_ATTR_CHARSET;
 		utf8_set(&gc->data, SIMPLE_BORDERS[cell_type]);
 		break;
+	case PANE_LINES_SPACES:
+		gc->attr &= ~GRID_ATTR_CHARSET;
+		utf8_set(&gc->data, ' ');
+		break;
 	default:
 		gc->attr |= GRID_ATTR_CHARSET;
 		utf8_set(&gc->data, CELL_BORDERS[cell_type]);

tmux.1

@@ -5078,6 +5078,8 @@ heavy lines using UTF-8 characters
 simple ASCII characters
 .It number
 the pane number
+.It spaces
+space characters
 .El
 .Pp
 .Ql double

tmux.h

@@ -994,7 +994,8 @@ enum pane_lines {
 	PANE_LINES_DOUBLE,
 	PANE_LINES_HEAVY,
 	PANE_LINES_SIMPLE,
-	PANE_LINES_NUMBER
+	PANE_LINES_NUMBER,
+	PANE_LINES_SPACES
 };
 
 /* Pane border indicator option. */