STR:
  - Go to https://mozilla.github.io/pdf.js/web/viewer.html
  - Click on "Open File" and open a pdf

The `Download` button and the `Current view` one just disappear.
I can reproduce this bug with Firefox nightly, Chrome stable and Epiphany on debian.
constructor() {
    throw new Error("Cannot initialize DefaultExternalServices.");
  }
  static updateFindControlState(data) {}
  static updateFindMatchesCount(data) {}
  static initPassiveLoading(callbacks) {}
  static async fallback(data) {}
  static reportTelemetry(data) {}
  static createDownloadManager(options) {
    throw new Error("Not implemented: createDownloadManager");
  }
  static createPreferences() {
    throw new Error("Not implemented: createPreferences");
  }
  static createL10n(options) {