Form elements defined outside hierarchy with form='form-id' are not captured by form.elements

Previous issues (#765 #2368) have addressed this issue resulting in PR #2369 but this fix does not add form elements defined outside form hierarchy with form='form-id' in the form.elements member.

Basic info:

• Node.js version: 10.15.3
• jsdom version: 15.1.1

Minimal reproduction case

const { JSDOM } = require('jsdom');

let form = `
    <form id='test-form'></form>
    <input type='text' name='test' form='test-form' />
`;

let dom = new JSDOM(form);
console.log('expecting one form element');
console.log('actual: ' + dom.window.document.forms[0].elements.length); // 0

Repro: https://github.com/FThompson/jsdom-form-attribute-bug

How does similar code behave in browsers?

form.elements detects elements defined outside form hierarchy with form attribute: https://jsfiddle.net/fthompson/2njuLq3r/2/

[ccwebdesign]

This seems to fix the issue, if you (JSDOM contributors) want to add it. It needs proper testing (sorry – low on time at the moment).

In master lib/jsdom/living/nodes/HTMLFormElement-impl.js:50

  get elements() {
    // TODO: Return a HTMLFormControlsCollection
    return HTMLCollection.createImpl([], {
      element: this,
-     query: () => domSymbolTree.treeToArray(this, {
+     query: () => domSymbolTree.treeToArray(this.ownerDocument, {
-       filter: node => isListed(node) && (node._localName !== "input" || node.type !== "image")
+       filter: node => isListed(node) && (node._localName !== "input" || node.type !== "image") && this == node.form
      })
    });
  }

[timrobinson33]

I’ve raised another bug which I think could be related https://github.com/jsdom/jsdom/issues/2979. - the form validation also excludes controls that aren’t physically nested inside the form