Tested in latest Chrome and FF:

If you have multiple PDF files embedded as Iframe (via viewer.html) and at least one of them is password protected, the browser automatically jumps to the (first) iframe that shows the password prompt.

Imho, the iframe shouldn't modify the behavior of it's parent in any way.

See https://support.softcreatr.com/thread/4360-pdf-test/ for a demo.
"pdfkit",
  "itext",
  "prince",
  "quarkxpress",
  "mac os x",
  "microsoft",
  "openoffice",
  "oracle",
  "luradocument",
  "pdf-xchange",
  "antenna house",
  "aspose.cells",
  "fpdf",
];
class DefaultExternalServices {
  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) {