ViewportAnchor anchor

The end of the viewport from which the paint offset is computed.

See ViewportAnchor for more detail.

Source 

ViewportAnchor get anchor => _anchor;
void anchor=(ViewportAnchor value)

Source 

set anchor(ViewportAnchor value) {
  assert(value != null);
  if (value == _anchor)
    return;
  _anchor = value;
  markNeedsPaint();
  markNeedsSemanticsUpdate();
}