bug#74274: [PATCH] Revert part of d3f8ed730f to avoid segmentation fault

[Eli Zaretskii <eliz@gnu.org>]

> Cc: Gerd Möllmann <gerd@gnu.org>, 74274@debbugs.gnu.org
> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Date: Sat, 09 Nov 2024 08:23:44 +0100
> 
> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
> 
> > qijian gong <gongqijian@gmail.com> writes:
> >
> >> Gerd Möllmann <gerd.moellmann@gmail.com>于2024年11月9日 周六下午12:15写道：
> >>
> >>  Do I understand this right, that the advice you add to
> >>  tty-tip-compute-position only serves the purpose of being able to pop up
> >>  a tip frame early, when mouse-position doesn't really have a position to
> >>  report? Or does it also serve another purpose?
> >>
> >> Yes, that's correct, there is no other purpose. It seems
> >> mouse-position doesn't really have a position until the mouse was
> >> moved by user.
> >
> > Ok, thanks.
> >
> > I think I know what this is. It's the very particular case of the very
> > first redisplay + presence of a child freame, which copies glyphs from a
> > current matrix that is still clear, i.e. zeroed, so that glyph::frame
> > is zero and so on.
> 
> Which is fixed for me with
> 
> 1 file changed, 6 insertions(+), 3 deletions(-)
> src/dispnew.c | 9 ++++++---
> 
> modified   src/dispnew.c
> @@ -3544,9 +3544,12 @@ prepare_desired_root_row (struct frame *root, int y)
>    if (!root_row->enabled_p)
>      {
>        struct glyph_row *from = MATRIX_ROW (root->current_matrix, y);
> -      memcpy (root_row->glyphs[0], from->glyphs[0],
> -	      root->current_matrix->matrix_w * sizeof (struct glyph));
> -      root_row->enabled_p = true;
> +      if (from->enabled_p)
> +	{
> +	  memcpy (root_row->glyphs[0], from->glyphs[0],
> +		  root->current_matrix->matrix_w * sizeof (struct glyph));
> +	  root_row->enabled_p = true;
> +	}
>      }
>    return root_row;
>  }

Should this perhaps have an eassert which verifies that every glyph
has a valid frame pointer?  At the very least please add a comment
there explaining the need for the enabled_p test and mentioning the
frame pointer of the glyphs.

Thanks.