Fix vertical-motion across the place where line-number width changes

* src/indent.c (line_number_display_width): New function,
refactored from line-number width calculations in vertical-motion.
(Fvertical_motion): Call line_number_display_width when the width
of line-number display is needed.
(Fline_number_display_width): New defun.
(syms_of_indent): Defsubr it.

* doc/lispref/display.texi (Size of Displayed Text): Document
line-number-display-width.

* etc/NEWS: Mention line-number-display-width.

* lisp/simple.el (last--line-number-width): New internal variable.
(line-move-visual): Use it to adjust temporary-goal-column when
line-number display changes its width.

1 files changed, 62 insertions, 22 deletions

diff --git a/src/indent.c b/src/indent.c
index 70351f90466..ba936509934 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -1947,6 +1947,59 @@ vmotion (register ptrdiff_t from, register ptrdiff_t from_byte,
                          -1, hscroll, 0, w);
 }
 
+/* Return the width taken by line-number display in window W.  */
+static void
+line_number_display_width (struct window *w, int *width, int *pixel_width)
+{
+  if (NILP (Vdisplay_line_numbers))
+    {
+      *width = 0;
+      *pixel_width = 0;
+    }
+  else
+    {
+      struct it it;
+      struct text_pos wstart;
+      bool saved_restriction = false;
+      ptrdiff_t count = SPECPDL_INDEX ();
+      SET_TEXT_POS_FROM_MARKER (wstart, w->start);
+      void *itdata = bidi_shelve_cache ();
+      /* We must start from window's start point, but it could be
+         outside the accessible region.  */
+      if (wstart.charpos < BEGV || wstart.charpos > ZV)
+        {
+          record_unwind_protect (save_restriction_restore,
+                                 save_restriction_save ());
+          Fwiden ();
+          saved_restriction = true;
+        }
+      start_display (&it, w, wstart);
+      move_it_by_lines (&it, 1);
+      *width = it.lnum_width;
+      *pixel_width = it.lnum_pixel_width;
+      if (saved_restriction)
+        unbind_to (count, Qnil);
+      bidi_unshelve_cache (itdata, 0);
+    }
+}
+
+DEFUN ("line-number-display-width", Fline_number_display_width,
+       Sline_number_display_width, 0, 1, 0,
+       doc: /* Return the width used for displaying line numbers in the selected window.
+If optional argument PIXELWISE is non-nil, return the width in pixels,
+otherwise return the width in columns of the face used to display
+line numbers, `line-number'.  */)
+  (Lisp_Object pixelwise)
+{
+  int width, pixel_width;
+  line_number_display_width (XWINDOW (selected_window), &width, &pixel_width);
+  if (!NILP (pixelwise))
+    return make_number (pixel_width);
+  /* FIXME: The "+ 2" part knows that we add a blank on each side of
+     the line number when producing glyphs for display.  */
+  return make_number (width + 2);
+}
+
 /* In window W (derived from WINDOW), return x coordinate for column
    COL (derived from COLUMN).  */
 static int
@@ -2073,30 +2126,10 @@ whether or not it is currently displayed in some window.  */)
          that's what normal window redisplay does.  Otherwise C-n/C-p
          will sometimes err by one column.  */
       int lnum_width = 0;
+      int lnum_pixel_width = 0;
       if (!NILP (Vdisplay_line_numbers)
           && !EQ (Vdisplay_line_numbers, Qvisual))
-        {
-          struct text_pos wstart;
-          bool saved_restriction = false;
-          ptrdiff_t count1 = SPECPDL_INDEX ();
-          SET_TEXT_POS_FROM_MARKER (wstart, w->start);
-          itdata = bidi_shelve_cache ();
-          /* We must start from window's start point, but it could be
-             outside the accessible region.  */
-          if (wstart.charpos < BEGV || wstart.charpos > ZV)
-            {
-              record_unwind_protect (save_restriction_restore,
-                                     save_restriction_save ());
-              Fwiden ();
-              saved_restriction = true;
-            }
-          start_display (&it, w, wstart);
-          move_it_by_lines (&it, 1);
-          lnum_width = it.lnum_width;
-          if (saved_restriction)
-            unbind_to (count1, Qnil);
-          bidi_unshelve_cache (itdata, 0);
-        }
+        line_number_display_width (w, &lnum_width, &lnum_pixel_width);
       SET_TEXT_POS (pt, PT, PT_BYTE);
       itdata = bidi_shelve_cache ();
       start_display (&it, w, pt);
@@ -2277,6 +2310,12 @@ whether or not it is currently displayed in some window.  */)
          an addition to the hscroll amount.  */
       if (lcols_given)
         {
+          /* If we are displaying line numbers, we could cross the
+             line where the width of the line-number display changes,
+             in which case we need to fix up the pixel coordinate
+             accordingly.  */
+          if (lnum_pixel_width > 0)
+            to_x += it.lnum_pixel_width - lnum_pixel_width;
           move_it_in_display_line (&it, ZV, first_x + to_x, MOVE_TO_X);
           /* If we find ourselves in the middle of an overlay string
              which includes a newline after current string position,
@@ -2322,6 +2361,7 @@ syms_of_indent (void)
   defsubr (&Sindent_to);
   defsubr (&Scurrent_column);
   defsubr (&Smove_to_column);
+  defsubr (&Sline_number_display_width);
   defsubr (&Svertical_motion);
   defsubr (&Scompute_motion);
 }