code: func NewFlex() *Flex {
  	f := &Flex{
  		Box:       NewBox().SetBackgroundColor(tcell.ColorDefault),
  		direction: FlexColumn,
  	}
  	f.focus = f
  	return f
  }
docs: // NewFlex returns a new flexbox layout container with no primitives and its
  // direction set to FlexColumn. To add primitives to this layout, see AddItem().
  // To change the direction, see SetDirection().
  //
  // Note that Box, the superclass of Flex, will have its background color set to
  // transparent so that any nil flex items will leave their background unchanged.
  // To clear a Flex's background before any items are drawn, set it to the
  // desired color:
  //
  //   flex.SetBackgroundColor(tview.Styles.PrimitiveBackgroundColor)