Spec

https://html.spec.whatwg.org/multipage/interaction.html#focusing-steps (the 5th entry under focusing step 1)

If new focus target is a browsing context container with a non-null nested browsing context
Let new focus target be the browsing context container's nested browsing context's active document.

https://www.w3.org/TR/html5/editing.html#focus

When a child browsing context is focused, its browsing context container must also have focus.

Test

https://github.com/w3c/web-platform-tests/blob/master/html/editing/focus/document-level-focus-apis/document-level-apis.html

Required changes

In lib/jsdom/living/helpers/focusing.js:

• exports.isFocusableAreaElement should return true iframe element. Could use something like:

  if (elImpl instanceof elImpl._ownerDocument._defaultView.HTMLIFrameElement) {
    return true;
  }

(using the defaultView's HTMLIFrameElement constructor means that if nested browsing contexts are ever made separate / sandboxed, the check will still work)

In lib/jsdom/living/nodes/HTMLElement-impl.js:

• focus needs to recursively check ownerDocument.defaultView.parent and invoke .focus() on any parent HTMLIFrameElement:

    if (this._ownerDocument._defaultView._frameElement) {
      this._ownerDocument._defaultView._frameElement.focus();
    }

• blur shouldn’t need to change, I don’t think. When a focusable element inside an iframe is blurred, the iframe element itself (the browsing context container) is still the activeElement of its parent.