fit property
How to size the non-positioned children in the stack.
The constraints passed into the RenderStack from its parent are either loosened (StackFit.loose) or tightened to their biggest size (StackFit.expand).
Implementation
StackFit get fit => _fit;Implementation
set fit(StackFit value) {
  assert(value != null);
  if (_fit != value) {
    _fit = value;
    markNeedsLayout();
  }
}