Avoid off-by-one errors in column C-n/C-p calculations

* src/indent.c (Fvertical_motion): Help C-n/C-p estimate correctly
the width used up by line numbers by looking near the window-start
point.

1 files changed, 18 insertions, 1 deletions

diff --git a/src/indent.c b/src/indent.c
index adecc3622a8..2cacfbbe3c0 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -2068,9 +2068,26 @@ whether or not it is currently displayed in some window.  */)
           start_x = window_column_x (w, window, start_col, cur_col);
         }
 
-      itdata = bidi_shelve_cache ();
+      /* When displaying line numbers, we need to prime IT's
+         lnum_width with the value calculated at window's start, since
+         that's what normal window redisplay does.  Otherwise C-n/C-p
+         will sometimes err by one column.  */
+      int lnum_width = 0;
+      if (!NILP (Vdisplay_line_numbers)
+          && !EQ (Vdisplay_line_numbers, Qvisual))
+        {
+          struct text_pos wstart;
+          SET_TEXT_POS_FROM_MARKER (wstart, w->start);
+          itdata = bidi_shelve_cache ();
+          start_display (&it, w, wstart);
+          move_it_by_lines (&it, 1);
+          lnum_width = it.lnum_width;
+          bidi_unshelve_cache (itdata, 0);
+        }
       SET_TEXT_POS (pt, PT, PT_BYTE);
+      itdata = bidi_shelve_cache ();
       start_display (&it, w, pt);
+      it.lnum_width = lnum_width;
       first_x = it.first_visible_x;
       it_start = IT_CHARPOS (it);