resize method
      
  
void
resize(})
       
    
override
    Resizes the controller by the given delta amount.
Implementation
@override
void resize(double newSize, double paneSize,
    {double? minSize, double? maxSize}) {
  _flex = newSize.clamp(minSize ?? 0, maxSize ?? double.infinity) / paneSize;
  notifyListeners();
}