Display problems with `before-string' in overlay

[Richard Stallman <rms@gnu.org>]

Most of your proposed change in xdisp.c is the addition of the OVERLAY
argument to `string_buffer_position'.  That is harmless.

It includes two substantial changes which could really alter Emacs's
behavior.  I've included them below.

Could you explain them more clearly, in a way that demonstrates
they are correct?



***************
*** 11933,11952 ****
        Lisp_Object string;
        struct glyph *stop = glyph;
        int pos;
  
        limit = make_number (pt_old + 1);
        glyph = string_start;
        x = string_start_x;
        string = glyph->object;
!       pos = string_buffer_position (w, string, string_before_pos);
!       /* If STRING is from overlay, LAST_POS == 0.  We skip such glyphs
! 	 because we always put cursor after overlay strings.  */
!       while (pos == 0 && glyph < stop)
  	{
  	  string = glyph->object;
  	  SKIP_GLYPHS (glyph, stop, x, EQ (glyph->object, string));
  	  if (glyph < stop)
! 	    pos = string_buffer_position (w, glyph->object, string_before_pos);
  	}
  
        while (glyph < stop)
--- 11934,11955 ----
        Lisp_Object string;
        struct glyph *stop = glyph;
        int pos;
+       Lisp_Object overlay = Qnil;
  
        limit = make_number (pt_old + 1);
        glyph = string_start;
        x = string_start_x;
        string = glyph->object;
!       pos = string_buffer_position (w, string, string_before_pos, &overlay);
!       /* If STRING is from overlay, skip its glyphs because we always
! 	 put cursor after overlay strings.  */
!       while ((pos == 0 || !NILP (overlay)) && glyph < stop)
  	{
  	  string = glyph->object;
  	  SKIP_GLYPHS (glyph, stop, x, EQ (glyph->object, string));
  	  if (glyph < stop)
! 	    pos = string_buffer_position (w, glyph->object,
! 					  string_before_pos, overlay);
  	}
  
        while (glyph < stop)

***************
*** 15854,15865 ****
    if (PT == MATRIX_ROW_END_CHARPOS (row))
      {
        /* If the row ends with a newline from a string, we don't want
! 	 the cursor there, but we still want it at the start of the
! 	 string if the string starts in this row.
  	 If the row is continued it doesn't end in a newline.  */
        if (CHARPOS (row->end.string_pos) >= 0)
! 	cursor_row_p = (row->continued_p
! 			|| PT >= MATRIX_ROW_START_CHARPOS (row));
        else if (MATRIX_ROW_ENDS_IN_MIDDLE_OF_CHAR_P (row))
  	{
  	  /* If the row ends in middle of a real character,
--- 15857,15866 ----
    if (PT == MATRIX_ROW_END_CHARPOS (row))
      {
        /* If the row ends with a newline from a string, we don't want
! 	 the cursor there.
  	 If the row is continued it doesn't end in a newline.  */
        if (CHARPOS (row->end.string_pos) >= 0)
! 	cursor_row_p = row->continued_p;
        else if (MATRIX_ROW_ENDS_IN_MIDDLE_OF_CHAR_P (row))
  	{
  	  /* If the row ends in middle of a real character,