an overscroll. <code>scrollChildren</code> is the count of the
total number of child nodes that contribute semantics and <code>scrollIndex</code>
is the index of the first visible child node that contributes semantics.</p>
<p>The <code>rect</code> is the region occupied by this node in its own coordinate
system.</p>
<p>The <code>transform</code> is a matrix that maps this node's coordinate system into
its parent's coordinate system.</p>
<p>The <code>elevation</code> describes the distance in z-direction bet