screen: commit a wide cell's tail to prev, or the diff leaves a hole

A wide glyph's tail column is never painted or compared, and prev never
learned it either. Overwrite the head later and the terminal blanks the
tail cell -- but if that column's new content matches the stale prev, the
diff skips it, leaving a one-cell hole where the glyph's second half was.

Seen as a missing rule cell in a pane's top border after spawning a new
tab: the old tab's centered title started with a wide glyph, the new
tab's border put the rule back, and the tail column stayed blank.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/screen.c b/src/screen.c
index b768162..79a81a9 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -483,6 +483,16 @@ void screen_render(screen_t *s) {
 
       *p = *c;
       s->prev_link[idx] = s->cur_link[idx];
+      /* A wide cell owns two columns but only its head is ever compared or
+       * painted, so commit the tail to prev here as well. Leave it stale and
+       * a later frame that puts back what that column used to hold skips it
+       * as unchanged -- while the terminal blanked it the moment the head
+       * was overwritten. The difference is a one-cell hole in a border,
+       * sitting where a wide glyph's second half used to be. */
+      if (c->width == 2 && (size_t)x + 1 < s->cols) {
+        s->prev[idx + 1] = s->cur[idx + 1];
+        s->prev_link[idx + 1] = s->cur_link[idx + 1];
+      }
       cx += c->width ? c->width : 1;
       if (cx >= s->cols) have_pos = false; /* wrap is terminal-dependent */
     }