constructor(parent, reader = undefined) {
    this.parent_ = parent;
    this.reader_ = reader;
    this.container_ = this.newContainer_();
    /** @type {?number} Id for automatic hiding timeout. */
    this.timeout_ = null;
    /** @type {number} Fadeout delay (for tests to control). */
    this.fadeout_ = 200;
  }
@param {!Element} parent The node that we will display inside.
@param {?hterm.AccessibilityReader=} reader Helper for reading content.