message: simplify className logic in UnderlineNav (again)
diff: @@ -18,20 +18,20 @@ export default function UnderlineNav(props) {
  }
  const {className = '', selected} = child.props
  - const newProps = {className}
  + const newProps = {}
  // add the ITEM_CLASS to all children without one
  if (!className || !className.includes(ITEM_CLASS)) {
  newProps.className = classnames(ITEM_CLASS, className)
  }
  if (selected === true && !className.includes(SELECTED_CLASS)) {
  - newProps.className = classnames(newProps.className, SELECTED_CLASS)
  + newProps.className = classnames(newProps.className || className, SELECTED_CLASS)
  }
  // if this is a react-router NavLink (duck typing!),
  // set activeClassName={SELECTED_CLASS}
  if (child.type.name === 'NavLink') {
  newProps.activeClassName = SELECTED_CLASS
  }
  - return Object.keys(newProps).length ? React.cloneElement(child, newProps) : child
  + return React.cloneElement(child, newProps)
  })
  return (
  