Before scroll:
text "ABC DEF GHI JKL MNO PQR STU"
  bounding_box: [0,0 70x40]
  fragment_visible_bounding_boxes:
    [0,0 70x10]
    [0,10 70x10]
    [0,20 70x10]
    [0,30 30x10]

After scroll:
text "ABC DEF GHI JKL MNO PQR STU"
  outer_bounding_box: [-15,-15 70x40]
  visible_bounding_box: [0,0 55x25]
  fragment_visible_bounding_boxes:
    [0,0 55x5]
    [0,5 55x10]
    [0,15 15x10]