Clear pane separator cells in box mode

When box mode is active, the separator columns and rows between panes
were not being cleared, causing old border characters to persist.
Each pane's box is drawn within its allocated space, but the layout
system places separator cells between panes that were left untouched.

Add code to clear:
- The separator column immediately to the right of each pane
- The separator row immediately below each pane
- The corner cell where separators intersect

screen-redraw.c

@@ -986,6 +986,37 @@ screen_redraw_draw_active_box(struct screen_redraw_ctx *ctx)
 				tty_cell(tty, &blank_gc, &grid_default_cell, NULL, NULL);
 			}
 		}
+
+		/*
+		 * Clear separator column to the right of this pane. This is
+		 * the column that normally holds the pane separator border.
+		 */
+		if (wp->xoff + wp->sx < w->sx) {
+			for (i = top; i <= bottom; i++) {
+				tty_cursor(tty, wp->xoff + wp->sx, tty_top + i);
+				tty_cell(tty, &blank_gc, &grid_default_cell,
+				    NULL, NULL);
+			}
+		}
+
+		/*
+		 * Clear separator row below this pane. This is the row that
+		 * normally holds the pane separator border.
+		 */
+		if (wp->yoff + wp->sy < w->sy) {
+			for (i = left; i <= right; i++) {
+				tty_cursor(tty, i, tty_top + wp->yoff + wp->sy);
+				tty_cell(tty, &blank_gc, &grid_default_cell,
+				    NULL, NULL);
+			}
+			/* Also clear the corner where separators meet. */
+			if (wp->xoff + wp->sx < w->sx) {
+				tty_cursor(tty, wp->xoff + wp->sx,
+				    tty_top + wp->yoff + wp->sy);
+				tty_cell(tty, &blank_gc, &grid_default_cell,
+				    NULL, NULL);
+			}
+		}
 	}
 }