springConfig = key => {
  switch (key) {
    case 'opacity': {
      return {
        tension: 170,
        friction: 22,
      };
    }
    case 'translateY': {
      return {
        tension: 95,
        friction: 25,
      };
    }
    default: {
      return config.default;
    }
  }
}
TODO: This code duplicates a lot of TwoPaneModal. Should be reconciled :/

TODO: Accessibility. There are a lot of a11y things for modals, and this
component isn't doing any of them :/

TODO: Use a portal to render this at the top-level, to avoid any z-index
issues?