implemented by VerticalListFixedRowLayout
The primary direction that this layout is expected to scroll. Not all layouts will necessarily have a primary direction.
Determines if the container calls
layout() when the scroll position
changes. Useful for transforming items as the view port scrolls. This
alue should typically be
true for layouts that implement the
IVirtualLayout interface and the
useVirtualLayout property is set to
true. May also be used by layouts that toggle item visibility as the
items scroll into and out of the view port.