Filed by: hskupin [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=360489909&repo=try
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/HrAZcTGkQBSlLfh58cduJQ/runs/0/artifacts/public/logs/live_backing.log

[task 2021-12-08T08:24:23.235Z] 08:24:23 INFO - TEST-UNEXPECTED-FAIL | /webdriver/tests/execute_script/collections.py | test_shadow_root - AssertionError: unknown error (500): TypeError: can't access property "documentElement", node.ownerDocument is null
[task 2021-12-08T08:24:23.235Z] 08:24:23 INFO - session = <Session 5d9ea162-ab8a-41dc-b966-1c0bd2ca4f83>
[task 2021-12-08T08:24:23.235Z] 08:24:23 INFO - inline = <function inline.<locals>.inline at 0x7f10bbd11d08>
[task 2021-12-08T08:24:23.235Z] 08:24:23 INFO -
[task 2021-12-08T08:24:23.235Z] 08:24:23 INFO - def test_shadow_root(session, inline):
[task 2021-12-08T08:24:23.235Z] 08:24:23 INFO - session.url = inline("""
[task 2021-12-08T08:24:23.235Z] 08:24:23 INFO - <style>
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - custom-checkbox-element {
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - display:block; width:20px; height:20px;
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - }
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - </style>
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - <custom-checkbox-element></custom-checkbox-element>
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - <script>
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - customElements.define('custom-checkbox-element',
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - class extends HTMLElement {
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - constructor() {
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - super();
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - this.attachShadow({mode: 'open'}).innerHTML = [task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - <div><input type="checkbox"/></div> [task 2021-12-08T08:24:23.236Z] 08:24:23 INFO -;
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - }
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - });
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - </script>""")
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - custom_element = session.find.css("custom-checkbox-element", all=False)
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - expected = custom_element.shadow_root
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - response = execute_script(session,
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - "return document.querySelector('custom-checkbox-element').shadowRoot")
[task 2021-12-08T08:24:23.236Z] 08:24:23 INFO - > value = assert_success(response)

Note that the same test but for `Execute Async Script` works just fine.

Here is a simple test case for Marionette that reproduces the problem. And again only for sandbox=None and not for Execute Async Script:

    def test_case(self):
        page = inline("""
        <style>
            custom-checkbox-element {
                display:block; width:20px; height:20px;
            }
        </style>
        <custom-checkbox-element id="checkbox"></custom-checkbox-element>
        <script>
            customElements.define('custom-checkbox-element',
                class extends HTMLElement {
                    constructor() {
                            super();
                            this.attachShadow({mode: 'open'}).innerHTML = `
                                <div><input type="checkbox"/></div>
                            `;
                        }
                });
        </script>""")
        self.marionette.navigate(page)

        self.marionette.execute_async_script("""
            const resolve = arguments[0];
            resolve(document.querySelector('custom-checkbox-element').shadowRoot);
        """, sandbox=None)

        self.marionette.execute_script("""
            return document.querySelector('custom-checkbox-element').shadowRoot;
        """, sandbox=None)

The exact stack of this failure is:

1639382595392 Marionette DEBUG 3 <- [1,4,{"error":"unknown error","message":"TypeError: can't access property "documentElement", node.ownerDocument is null","stacktrace":"element.isInXULDocument@chrome://remote/content/marionette/element.js:1353:1\nfrom@chrome://remote/content/marionette/element.js:1497:19\nelement.getElementId@chrome://remote/content/marionette/element.js:624:28\nevaluate.toJSON@chrome://remote/content/marionette/evaluate.js:348:20\nevaluate.toJSON@chrome://remote/content/marionette/evaluate.js:362:27\nreceiveMessage@chrome://remote/content/marionette/actors/MarionetteCommandsChild.jsm:177:31\n"},null]

The first step here would be to fix the check at:
https://searchfox.org/mozilla-central/rev/4646b826a25d3825cf209db890862b45fa09ffc3/remote/marionette/element.js#1350-1353

As it looks like there are cases when ownerDocument can be null and that's causing the above failure. But when done so we will end-up with a cyclic error.

https://wiki.mozilla.org/Bug_Triage#Intermittent_Test_Failure_Cleanup
For more information, please visit auto_nag documentation.

This is not fixed but marked as failed so it's expected to not cause failures.

Updating the bug's summary because of the changes for the failure as triggered by landing the patch on bug 1775064.

With bug 1793920 fixed this will no longer be a stale element reference error but a javascript error (500): Cyclic object value.

With bug 1794078 the test is moved to arguments.py

Fixed by the patch on bug 1764594.