Before scroll:
text "ABC DEF GHI JKL MNO PQR STU VWX YZ 0123456789"
  bounding_box: [0,0 100x50]
  fragment_visible_bounding_boxes:
    [0,0 70x10]
    [0,10 70x10]
    [0,20 70x10]
    [0,30 100x10]
    [0,40 100x10]

After scroll:
text "ABC DEF GHI JKL MNO PQR STU VWX YZ 0123456789"
  outer_bounding_box: [-15,-15 100x50]
  visible_bounding_box: [0,0 85x35]
  fragment_visible_bounding_boxes:
    [0,0 55x5]
    [0,5 55x10]
    [0,15 85x10]
    [0,25 85x10]