async extAdded (win, ext) {
    if (!ext) {
      return undefined
    }
    let cbCalled = false

    /**
     *
     * @returns {void}
     */
    const runCallback = () => {
      if (ext.callback && !cbCalled) {
        cbCalled = true
        ext.callback.call(this)
      }
    }

    if (ext.events) {
      this.leftPanel.add(ext.events.id, ext.events.click)
    }
    return runCallback()
  }
@param {external:Window} win
@param {module:svgcanvas.SvgCanvas#event:extension_added} ext
@listens module:svgcanvas.SvgCanvas#event:extension_added
@returns {Promise<void>|void} Resolves to `undefined`