title: How is the area of the scrollable overflow rectangle calculated?
body: <p>I don’t understand the description of the scrollable overflow area in [css-overflow-3]:</p>
  <p>The scrollable overflow area is the union of:</p>
  <ul>
  <li>the box’s own content and padding areas</li>
  <li>all line boxes directly contained by the box</li>
  <li>the border boxes of all boxes for which it is the containing block and whose border boxes are positioned not wholly outside its block-start or inline-start padding edges, accounting for transforms by projecting each box onto the plane of the element that establishes its 3D rendering context. [CSS3-TRANSFORMS]</li>
  <li>......</li>
  </ul>
  <p>Can anyone help me explain?<br />
  Document address：https://www.w3.org/TR/css-overflow-3/#scrollable</p>