def initial_landing_distance(interest_area, fixation_sequence):
    """

    Given an interest area and fixation sequence, return the initial landing
    distance on that interest area. The initial landing distance is the pixel
    distance between the first fixation to land in an interest area and the
    left edge of that interest area (or, in the case of right-to-left text,
    the right edge). Technically, the distance is measured from the text onset
    without including any padding. Returns `None` if no fixation landed on the
    interest area.

    """
    for fixation in fixation_sequence.iter_without_discards():
        if fixation in interest_area:
            for char in interest_area:
                if fixation in char:  # be sure not to find a fixation in the padding
                    return abs(interest_area.onset - fixation.x)
    return None