// complicated:
// 1. if scrolling in one direction only, scroll only
// those children that intersect shaft defined by the rectangle
// and scrolling direction
// 2. if scrolling in both axes, scroll all children
// complicated:
// 1. if scrolling in one direction only, scroll only
// those children that intersect shaft defined by the rectangle
// and scrolling direction
// 2. if scrolling in both axes, scroll all children