setCollapsible(collapsible) {
    if (this.collapsible_ === collapsible) {
      return;
    }
    this.collapsible_ = collapsible;
    this.element.classList.toggle('ol-uncollapsible');
    if (this.userCollapsed_) {
      this.handleToggle_();
    }
  }
Set whether the attribution should be collapsible.
@param {boolean} collapsible True if the widget is collapsible.
@api