Called by RenderVirtualViewport during layout.
Subclasses should override this function to compute materializedChildBase and materializedChildCount. Overrides should call this function to update the RenderVirtualViewport's paint offset and to materialize the children.
Source
void layout(BoxConstraints constraints) {
assert(startOffsetBase != null);
assert(startOffsetLimit != null);
_updatePaintOffset();
owner.buildScope(this, _materializeChildren);
}