_getCSSTop(y) {
    if (y === undefined || y === null) {
      return {
        top: 0
      };
    }

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