code: func (b *BoxLayout) HandleEvent(ev tcell.Event) bool {
  	switch ev.(type) {
  	case *EventWidgetContent:
  		// This can only have come from one of our children.
  		b.changed = true
  		b.PostEventWidgetContent(b)
  		return true
  	}
  	for _, c := range b.cells {
  		if c.widget.HandleEvent(ev) {
  			return true
  		}
  	}
  	return false
  }
docs: // HandleEvent implements a tcell.EventHandler.  The only events
  // we care about are Widget change events from our children. We
  // watch for those so that if the child changes, we can arrange
  // to update our layout.