Eli Zaretskii writes:

[...]
>
> Thanks.  One more thing, and then I need to think how this could
> happen:
>
>   (gdb) frame 3

#3  0x00005555555a000c in update_frame_line (f=0x555556113798, vpos=24, updating_menu_p=false) at dispnew.c:5089

>   
>   (gdb) print (desired_row->glyphs[1])[149]
>

$1 = {
  charpos = -1,
  object = XIL(0),
  pixel_width = 0,
  ascent = 0,
  descent = 0,
  voffset = 0,
  type = 0,
  multibyte_p = false,
  left_box_line_p = false,
  right_box_line_p = false,
  overlaps_vertically_p = false,
  padding_p = false,
  glyph_not_available_p = false,
  avoid_cursor_p = false,
  resolved_level = 0,
  bidi_type = 0,
  face_id = 0,
  font_type = 0,
  slice = {
    img = {
      x = 0,
      y = 0,
      width = 0,
      height = 0
    },
    cmp = {
      from = 0,
      to = 0
    },
    glyphless = {
      upper_xoff = 0,
      upper_yoff = 0,
      lower_xoff = 0,
      lower_yoff = 0
    }
  },
  u = {
    ch = 32,
    cmp = {
      automatic = false,
      id = 16
    },
    img_id = 32,
    stretch = {
      height = 32,
      ascent = 0
    },
    glyphless = {
      method = 0,
      for_no_font = false,
      len = 4,
      ch = 0
    },
    val = 32
  }
}


Thanks.