reposition (scrollable) {
    // Note: we use this for different kinds of popovers
    // typically, only context menus or alike require a 'relative' positioning
    if (this.state.position === 'relative') {
      const dtop = scrollable.getProperty('scrollTop') - this._initialScrollTop
      const dleft = scrollable.getProperty('scrollLeft') - this._initialScrollLeft
      this.el.css({
        top: this._topPos - dtop,
        left: this._leftPos + dleft
      })
    }
  }
ATTENTION: this method has to be called by the owner whenever the scrollable
has been scrolled.