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.

diff --git a/src/indent.c b/src/indent.c
index adecc3622a8a1a6a00e3203cf94a4f867b914b59..2cacfbbe3c019c1d6732618ae19d9edbb7636adf 100644 (file)
--- 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);