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  // For multipart forms (e.g., file uploads), jQuery Form targets the form

  // submission to an iframe instead of using an XHR object. The initial "src"

  // of the iframe, prior to the form submission, is set to options.iframeSrc.

  // "about:blank" is the semantically correct, standards-compliant, way to

  // initialize a blank iframe; however, some old IE versions (possibly only 6)

  // incorrectly report a mixed content warning when iframes with an

  // "about:blank" src are added to a parent document with an https:// origin.

  // jQuery Form works around this by defaulting to "javascript:false" instead,

  // but that breaks on Chrome 83, so here we force the semantically correct

  // behavior for all browsers except old IE.

  // @see https://www.drupal.org/project/drupal/issues/3143016

  // @see https://github.com/jquery-form/form/blob/df9cb101b9c9c085c8d75ad980c7ff1cf62063a1/jquery.form.js#L68

  // @see https://bugs.chromium.org/p/chromium/issues/detail?id=1084874

  // @see https://html.spec.whatwg.org/multipage/browsers.html#creating-browsing-contexts

  // @see https://developer.mozilla.org/en-US/docs/Web/Security/Same-origin_policy

  if (navigator.userAgent.indexOf("MSIE") === -1) {

    ajax.options.iframeSrc = 'about:blank';

  }