local s = string.sub(Lines[Cursor1.line].data, screen_line_starting_pos)

        local screen_line_starting_byte_offset = utf8.offset(Lines[Cursor1.line].data, screen_line_starting_pos)
        assert(screen_line_starting_byte_offset)
        local s = string.sub(Lines[Cursor1.line].data, screen_line_starting_byte_offset)

    local s = string.sub(Lines[Cursor1.line].data, new_screen_line_starting_pos)

    local new_screen_line_starting_byte_offset = utf8.offset(Lines[Cursor1.line].data, new_screen_line_starting_pos)
    assert(new_screen_line_starting_byte_offset)
    local s = string.sub(Lines[Cursor1.line].data, new_screen_line_starting_byte_offset)

    local s = string.sub(Lines[Cursor1.line].data, new_screen_line_starting_pos)

    local new_screen_line_starting_byte_offset = utf8.offset(Lines[Cursor1.line].data, new_screen_line_starting_pos)
    assert(new_screen_line_starting_byte_offset)
    local s = string.sub(Lines[Cursor1.line].data, new_screen_line_starting_byte_offset)

--?     print('iter', y, screen_line_index, screen_line_starting_pos, string.sub(line.data, screen_line_starting_pos))

      local screen_line_starting_byte_offset = utf8.offset(line.data, screen_line_starting_pos)
      assert(screen_line_starting_byte_offset)
--?     print('iter', y, screen_line_index, screen_line_starting_pos, string.sub(line.data, screen_line_starting_byte_offset))

      local screen_line_starting_byte_offset = utf8.offset(line.data, screen_line_starting_pos)
      assert(screen_line_starting_byte_offset)

-- static properties of the code
All strings are UTF-8. Bytes within them are not characters.
I try to label byte offsets as _offset, and character positions as _pos.
For example, string.sub should never use a _pos to substring, only an _offset.
Wish I had some static typing here. We're not going to try to write tests to catch issues like this.
-- manual tests