Fix bug #13277 with TTY cursor positioning in a line with overlay arrow.

 src/xdisp.c (set_cursor_from_row): Don't confuse a truncation or
 continuation glyph on a TTY with an indication of an empty line.
 src/dispextern.h: Improve commentary to glyph->charpos and
 glyph->object.

src/ChangeLog

+2012-12-27  Eli Zaretskii  <eliz@gnu.org>
+
+	* xdisp.c (set_cursor_from_row): Don't confuse a truncation or
+	continuation glyph on a TTY with an indication of an empty line.
+	(Bug#13277)
+
 2012-12-27  Glenn Morris  <rgm@gnu.org>
 	* cygw32.c (Fcygwin_convert_file_name_to_windows)

src/dispextern.h

@@ -317,13 +317,18 @@ struct glyph
      Lisp string, this is a position in that string.  If it is a
      buffer, this is a position in that buffer.  A value of -1
      together with a null object means glyph is a truncation glyph at
-     the start of a row.  */
+     the start of a row.  Right truncation and continuation glyphs at
+     the right edge of a row have their position set to the next
+     buffer position that is not shown on this row.  Glyphs inserted
+     by redisplay, such as the empty space after the end of a line on
+     TTYs, have this set to -1.  */
   ptrdiff_t charpos;
 
-  /* Lisp object source of this glyph.  Currently either a buffer or
-     a string, if the glyph was produced from characters which came from
+  /* Lisp object source of this glyph.  Currently either a buffer or a
+     string, if the glyph was produced from characters which came from
      a buffer or a string; or 0 if the glyph was inserted by redisplay
-     for its own purposes such as padding.  */
+     for its own purposes such as padding or truncation/continuation
+     glyphs on TTYs.  */
   Lisp_Object object;
 
   /* Width in pixels.  */

src/xdisp.c

@@ -14241,7 +14241,12 @@ set_cursor_from_row (struct window *w, struct glyph_row *row,
 	 CHARPOS is zero or negative.  */
       int empty_line_p =
 	(row->reversed_p ? glyph > glyphs_end : glyph < glyphs_end)
-	&& INTEGERP (glyph->object) && glyph->charpos > 0;
+	&& INTEGERP (glyph->object) && glyph->charpos > 0
+	/* On a TTY, continued and truncated rows also have a glyph at
+	   their end whose OBJECT is zero and whose CHARPOS is
+	   positive (the continuation and truncation glyphs), but such
+	   rows are obviously not "empty".  */
+	&& !(row->continued_p || row->truncated_on_right_p);
       if (row->ends_in_ellipsis_p && pos_after == last_pos)
 	{