def get_letter_index(self, mouse_x) -> int:
    """
    Returns possible letter-position of mouse.

    The function is independent from any specific line, so we could possible return a letter_index which
    is bigger than the letters in the line.
    Returns at least 0 to make sure it is possibly a valid index.
    """
    letter = int((mouse_x - self.xline_start) / self.letter_size_X)
    letter = 0 if letter < 0 else letter
    return letter