_getCSSBottom(y) {
    if (y === undefined || y === null) {
      return {
        bottom: 0
      };
    }

    const {innerHeight, marginBottom} = this.props;
    return {
      bottom: marginBottom + innerHeight - y
    };
  }
Get the bottom coordinate of the hint.
When y undefined or null, edge case, pin bottom.
@param {number} y Y.
@returns {{bottom: *}} Mixin.
@private